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http://www.widman.biz/uploads/Transaxle_oil.pdfQuoteThe Difference between GL-4 and GL-5 gear oils
by Richard Widman
Revision 6-2017
The original target audience for this paper was my group of friends in the Corvair world, but it applies
to all cars and is particularly important for all classic cars.
There is a lot of confusion about gear oils and the API classifications. In this paper I will try to
differentiate the two oils and clear up the mysteries that are flying all over the internet. It is extremely
common, or normal, for all GL-5 oils to claim they cover the API GL-4 requirements for gear oils. This
is a true statement. Does that make them satisfactory for synchromesh or synchronized transmissions?
NO! They meet the GEAR OIL specifications, not transmission oil specifications. The API GL-4 and
GL-5 categories do not mention or have anything to do with transmission synchronizers.
History:
The gear oils of a few decades ago had lead additives that were effective at wear reduction, but not very
good for the environment. A long time ago they began to be replaced by gear oils with a phosphorous
additive (in itself a decent anti-wear additive) with active sulfur to grip hold of the gears and create a
very solid sacrificial layer of material that could be worn off, thereby protecting the gear surface.
Eventually it was discovered that the active sulfur was causing corrosion of brass and other soft metals
used in differentials and transmissions.
Somewhere around 25 years ago a deactivated or buffered sulfur was developed that would react with
the phosphorous to create the protective/sacrificial layer in the conditions created in the gear boxes
(temperature and pressure) without being corrosive to the brass, copper, etc. This additive system is
used in most gear oils today.
The problems arise when we try or need to use the same product in the transmission that we use in the
differential. Many people have called oil companies and been told by the "Techs" that answer their
questions that their oils have buffered sulfur and therefore are not corrosive to yellow metals, so their
GL-5 oils can be used with brass components. While that answer is totally correct, it does not address
the question asked: Can I use your GL-5 in my synchromesh transmission?
Let's take a look at the API GL-5 rating. It is a rating for EP (Extreme Pressure) protection. The higher
the EP protection, the higher the GL category. In the mid 60's, Ford needed better protection in their
pickup trucks and GM developed the front wheel drive Oldsmobile Toronado that had a differential
with a very high angle of contact for power transmission to the wheels so a higher category was
developed (later to be called GL-6) to offer the protection needed. This level of protection can still be
claimed, but can no longer be tested since the Toronado rig used to test it is no longer available. (Note:
The 1966 and 1967 Toronados had sun gears between the axle shafts instead of spyder gears and a very
high offset, while suffering from the high temperatures of the engine compartment and very high
pressures.) This is why you will frequently see GL-6 listed as "obsolete". The test is obsolete, not the
car or its needs. Many other high performance cars continue to spec this level of EP performance.
In normal operation, the sulfur/phosphorous additive forms a black sacrificial coating on the gears and
anything it touches with a little pressure and temperature. As the gears turn, instead of wearing, the
sacrificial coating of additives is peeled off or worn off. This is normal and acceptable in all steel gears.
But when one or more of the surfaces is brass or another soft metal, the sacrificial coating is stronger
than the base metal, and instead of just peeling off, it takes with it a few microns of brass that it is
bound to.
The Difference between GL-4 and GL-5 gear oils 2 of 12
A traditional GL-4 gear oil of any given viscosity has about ½ of the level of sulfur/phosphorous
additive that would be in the GL-5 product, so the bond is not as strong, and therefore can be peeled off
without peeling a layer of brass (or less brass). This means that the GL-4 product provides a little less
extreme pressure protection, so in the differential of a high-powered car, it would not be the ideal
product in the differential. To understand this need we should be aware of the fact that the differential is
where the final torque is applied to the wheels (in most applications).
But in the transmission, we should consider two factors:
• Due to the fact that the differential applies the final torque, normally we do not need the full EP
protection in the transmission where less torque (about 30%) is applied.
• We need to be able to break the EP protection to stop the spinning of the gears long enough to
mesh them or synchronize them.
When we use a GL-5 product in a transmission that requires GL-4, we normally find 2 to 4 times as
much copper in the used oil as we would with a GL-4 product (with used oil analysis). Eventually the
synchronizers wear to the point that they no longer make contact with the other half of the cone,
bottoming out before stopping the opposing gear. (Refer to the picture below.)
GL-3
It should be noted that while GL-3 is frequently considered obsolete since it has less protection than the
GL-4, some transmission manufacturers today specify GL-3 (Chrysler and Mitsubishi among them).
There are many formulations of GL-3 oils in the market, some with sulfur/phosphorous additive and
some with zinc/phosphorous. Almost any diesel motor oil classifies as a GL-3 or GL-4 in gear
protection.
Synchromesh transmission oils
General Motors, Honda and others have developed oils that combine the best shift characteristics with
their transmission components for reduced wear. These products in general could classify as GL-4 oils
if they wanted to, but actually when we analyze their components they are very similar to 5W-30 diesel
motor oils, with a few friction modifiers added. The viscosity is closer to an ATF. They are way too thin
for a Corvair transmission or any other where a SAE 80W-90 or 75W-90 is recommended.
Modern transmission oils
I am getting questions about the application of traditional GL-4 or GL-5 oils in modern cars
or the use of modern transmission oils in classic cars, so let's look into them.
A lot of study has gone into the development of oils that are more slippery, more protective, and more
efficient. The current emphasis on fuel economy has driven many new transmission designs and oils to
match them. The economy gains have come with gear and machining design (smaller synchronizers
with double sided contact surfaces fused onto the ring) in an assembly that not only permits, but
requires, thinner oils. They also cannot tolerate sulfur/phosphorous additives in the oil, depending on
friction modifiers and new compounds to achieve GL-4 or better performance. This can become
confusing, as few lubricant manufacturers state their ingredients. All you see are claims for
performance or recommendations for their own brand. In most cases these new formulations are much
better for the transmission and its efficiency.
For the most part these are SAE 75W-80 and 75W-85 oils. Way too thin for most classic cars, but
necessary for modern cars. The additive package uses magnesium or calcium sulfonate compounds and
often synthetic base oil, and will give you better EP protection than a GL-4 sulfur/phosphorous oil.
The Difference between GL-4 and GL-5 gear oils 3 of 12
Many of these claim GL-4+ performance and pass most of the old GL-5 and SAE J2360 extreme
pressure tests, but additionally pass manufacturers tests for synchronization. The problems of where to
apply them lie with the auto manufacturers, where, for example, Toyota specifies in their owner's
manual, their SAE 75W without defining the rest of it. Oil sales people will sell you 75W-90, but what
Toyota is really asking for is 75W-85 without sulfur/phosphorous. From the comments I receive, a
standard SAE 75W-90 GL-4 does cause hard shifting and grinding in these Toyotas in 70ºF weather.
Nissan has the same problem with 75W-90 oil, but their manual clearly states 75W-80 for 6 speed
transmissions and 75W-85 for 5 speed transmissions. It does not specify "non sulfur/phosphorous" oil,
instead referring to "Genuine Nissan Oil" (which does not contain those additives).
VW calls for 75W oil that meets their spec. They are looking for a viscosity on the low end (or slightly
below) of a 75W-80 and no sulfur/phosphorous additives.
With these new non-sulfur/phosphorous oils giving a minimum of GL-4 extreme pressure protection, I
believe manufacturers will stop specifying GL-3 when they revise their manuals. Recommendations are
also coming out from the additive makers to use these new formulations in place of the older
Synchromesh Fluids.
Transaxles
So, what do we put in transaxles where the transmission and differential are combined in one unit? This
is a good question, and the answer lies in the design and surface area of the gears. If the contact
surfaces of the gears are big enough to carry the weight and torque necessary, we depend less on the oil
and its additives. If the surface area is compact, we need to depend more on the additive's ability to
handle the boundary lubrication. When we have transaxles, we have to depend on the manufacturer to
tell us what product is correct.
When we have transaxles, we also need to be aware of whether the differential portion uses Limited
Slip "LSD" technology of some kind that limits the slipping of the wheels in mud and snow. This
requires an additive that lets a clutch bind the wheel movement together at a certain speed differential
of the wheels. This additive can be in any oil. Many GL-5 oils have a small amount. This is often
enough for some differentials as long as they were not rinsed out. Some systems need more additive
than others. You can usually tell if you need more additive by making a U-Turn. If the inner wheels
click or try to lock up on the turn, you need more additive. Often an oil that has a little of this additive
will say it is satisfactory for "service fill" or "top-off".
How can you tell whether or not you have a limited slip differential? That is the easy part: Jack up both
driven wheels and spin one of them with your hand (transmission in neutral) if the other wheel spins
the same way, you have a limited slip differential. If it spins in the reverse direction, you do not. You
can also just jack up one wheel and try to spin it. If it refuses to turn, or turns with a lot of resistance,
and the car is in neutral, with no brake on, you have limited slip
You will find numerous wrong comments on forums and other sites where users claim that GL-4
means LSD. That is totally false. A check of the API site could have set them straight.
Engine/Transaxle combinations
While not used in any Corvairs, there are cars where the same oil sump is used for lubrication of the
engine, transmission, and differential. With this design, the parts of the transmission and differential are
dimensioned for use of a GL-4 (or perhaps at one time even a GL-3). My 1975 Morris Mini is an
The Difference between GL-4 and GL-5 gear oils 4 of 12
example of these cars. As far as I know, most have been fairly low HP engines, and where the higher
HP (turbo versions) were offered, synthetic oil was recommended. One caution that is not often
mentioned in those manuals: The multigrade oils used should be group II or stronger, or you should
avoid extending the interval beyond what is mentioned. The lesser quality multigrade oils tend to get
their polymers ground or sheered in the gears, just like some of the sludge prone OHV engines of today
where gears are used for synchronization of the cam and crank. Note: Corvair engines do not suffer
from this sheer due to the large diameter of the cam/crankshaft synchronization gears in the engine.
We can also note that the Chevrolet Luv pickups, as well as the Isuzu Rodeo and Trooper and many
others have recommended motor oil in the transmission for many years, even though they have
separate compartments. They only use gear oil in the differential.
The concept of using motor oil in these situations goes back to what I said towards the beginning. Most
motor oils can qualify for GL-4 (or at least GL-3) EP protection. As long as the torque on the gears in
low speed sliding action does not exceed what is covered by the GL-4, there is no benefit to
sulfur/phosphorous additives, but there is one for the synchronizers.
In the case of those single compartment motor/transaxle, there is an additional benefit of constantly
filtering the oil. The risk is that if you get gasoline in your oil you will thin out hydrodynamic film
protection in the gears. If you are running a mixture too rich, the additional soot (carbon particles) will
wear the gear surfaces as well as your camshaft.
Viscosity:
First it should be noted that the SAE motor oil viscosity chart (J300) is different from the SAE gear oil
chart (J306). A SAE 40 motor oil has approximately the same viscosity as one of the thinner SAE 90
gear oils, while the SAE 50 Motor oil is similar in viscosity to the thicker SAE 90 gear oils.
Viscosity has nothing to do with API Gear Oil classification (GL-4, GL-5, etc.). Some manual
transmissions specify an ATF (Automatic Transmission Fluid), others a SAE 75W-80 (almost the same
viscosity as the ATF), others a 75W-85, 75W-90 or 80W-90. Some specify a SAE 50 motor oil or SAE
50 CAT TO-4 transmission oil. The oil viscosity should be chosen according to the manufacturer's
recommendations for the temperatures where you will drive.
• If it is too thick (viscous) it will not displace fast enough from the synchronizers and will heat
up your transmission and cause hard shifts. It will also fail to flow properly in the needle
bearings or other tight spaces.
• If it is too thin it will not provide the hydrodynamic lubrication that is required between gears
and in the bearings or bushings.
If we look at the service manual for the Corvair, the one I have from 1960 recommends SAE 80 Gear
oil. While I do not have a SAE J306 viscosity chart earlier than the 1996 update, in the graph below we
can see what those viscosity limits were in 1996, with the blue being the range for SAE 80 and red
being the range for SAE 90. What this means is that any given brand had to stay within that range for
that weight of oil.
The Difference between GL-4 and GL-5 gear oils 5 of 12
You will note a huge range is possible between different brands within each range. In the chart on the
right-hand side you can see that the 80W-90 gear oils on the market today are thinner than the thinnest
SAE 90. You could actually use a synthetic 75W-140 if you could find it in a GL-4.
SAE 80W-90 or 85W-90 oils are often referred to as multigrade oils. This is not true. They have no
polymers to modify their viscosity. They are simply SAE 90 oils with a normal (95 to 105) viscosity
index where the low temperature properties either meet the 80W or 85W specification. So, an 80W-90
is at the low end of the SAE 90, while the 85W-90 is at the high end of the range for a SAE 90.
From personal experience and that of several friends who have tried them, I recommend staying away
from the 85W-90 in Corvairs and most other cars, since that puts it at the high end of the 90 scale. You
will notice a harder shift for the first few shifts of the day.
Recommendations
In general, you should use your manual. Nobody knows the vehicle more than the people who put it
together. The exceptions are cars like the Corvair where oils have changed dramatically since the
manuals were printed. There are other exceptions where those who write manuals have no idea how a
transmission works and are guided by wrong information from oil salespeople.
You need to realize that an oil with GL-4/GL-5 on the label is a GL-5. It is not appropriate for
synchronized transmissions.
The Difference between GL-4 and GL-5 gear oils 6 of 12
For the Corvair:
• Considering that most Corvairs have seen a lot of miles, there is probably some wear in the
transmission, so we could look at the higher viscosities within the recommendations unless it
has been rebuilt.
• This is a car that was produced long before any of the oils that are on the market today. My car
was a perfect example of what happens if you do not have any EP additives. The teeth on the
synchronizers were in excellent condition. But the rest of the transmission had severe signs of
wear. (See picture below)
• Too much additive may reduce the wear on gears, needle bearings, and shafts in certain
conditions, but will wear out your synchronizers. You need to look for a GL-4 oil that does not
include GL-5 in its label, even though it looks good, even though it claims GL-4/GL-5 on the
label. The Corvair differential is well built with parts much bigger than those of many cars
today. Those gears can carry the power of the car. You can see in the following picture of a
Toyota synchronizer how the brass synchronizer has worn so much that the entire ring bottoms
out and no longer grabs.
The Difference between GL-4 and GL-5 gear oils 7 of 12
• For many climates, the SAE 80W-90 is the best viscosity for the Corvair. But there are synthetic
products available that cover that full range with a SAE 75W-90 rating. This would be even
better especially in cooler climates. The trick in this formulation is a very high viscosity index
in the synthetic base oil. See the chart at the end of this article to see how a few typical oils
react to different temperatures. You should note that you might need to shift below the -20ºC
shown as a starting point in these graphs. In this case, the synthetic oil is your best bet. Most
manual automotive transmissions will run between 30ºC and 50ºC in normal operation. I have
been using 75W-90 GL-4 for at least a year and have much better shifts. Morning temperatures
here are normally in the 65ºF to 75ºF range.
• Corvairs with automatic transmissions: This is much simpler. Just use a good Dexron III in the
transmission and a good GL-5 in the differential, adding the LSD additive if the oil
manufacturer does not include it and if you have "positraction". They may or may not include
it. You have to read the label or spec sheet. As an example, I have
• 75W-80 GL-3 without LSD
• 75W-80 GL-4+ without LSD
• 75W-85 GL-4+ without LSD
• 75W-90 GL-4 with LSD and without
• 75W-90 GL-5 with LSD and without
• 80W-90 GL-4 with LSD and without
• 80W-90 GL-5 with LSD and without
• 75W-140 GL-5 Synthetic with and without LSD
• 85W-140 with and without LSD
The Difference between GL-4 and GL-5 gear oils 8 of 12
To confuse things a little more, we have to read the literature and sometimes consult with the
manufacturers. In looking for a few GL-4 products that would meet our needs, I identified Redline MT90
as a candidate, but then saw this on their website. I
questioned them and received this response.
"In your Corvair manual transaxle I would recommend the 75W90NS as a GL-5 type
gear oil was originally called for. A GL-4 gear oil is suitable for use in a zero or low
offset hypoid gear application, where extreme pressures of a high offset hypoid are
present a GL-5 gear oil is required. In your transaxle where high offset gears are not
present, a GL-4 gear oil could be used though we would recommend a GL-5 as called
for."
While he confirms that the Corvair does not have high offset hypoid gears ("In your transaxle where
high offset gears are not present"), he says he recommends his GL-5 product because the Corvair
originally recommended GL-5 ("a GL-5 type gear oil was originally called for"). There is a problem
with that statement: The GL categories are based on the ASTM STP 512 standards which were
originally developed in 1972, three years after the end of the production of the Corvair.
Here is a clarification from Lubrizol (one of the premier additive companies)
Summary
Just remember that GL-4 and GL-5 are gear oil ratings, not transmission oil ratings. Transmissions
have gears and synchronizers. These seemingly conflicting requirements must be met differently.
When someone tells you that their GL-5 covers GL-4, remember they are correct as far as EP
protection, but that is only half the answer. When they say their Sulfur/Phosphorous additive will not
corrode the yellow metals, they are also correct, but if there are enough to meet GL-5 protection, they
will slowly peel away your brass synchronizers.
What should you do if you cannot find a GL-4 that does not include GL-5 on its label? My next choice,
and one I almost did even though I have plenty of GL-4 80W-90, would be a SAE 50 oil that meets
CAT TO-4 specifications. My third choice would be a SAE 50 diesel motor oil of the highest API
classification I could find, preferably group II. As you will see on the graph below, the temperature
curve for a SAE 50 motor oil (or CAT transmission oil) and a SAE 80W-90 are very similar.
But we should not get carried away thinking we cannot find a GL-4 oil. There is plenty available in
75W-90. I get emails from all over the world, and we almost always come up with an answer.
What should you do if you've put a bigger engine in the Corvair or dramatically increased the HP? I
would use a synthetic GL-4 oil. If I felt it was not enough (gut feeling, stubbornness, etc.), I would
send it away for analysis, then use a GL-5 for the same period of time and analyze it. If the GL-4 is not
sufficient protection, it will show up in iron particles in the sample. The GL-5 might reduce the iron
The Difference between GL-4 and GL-5 gear oils 9 of 12
wear, but you can see for yourself how much copper increases. The only problem with this is the
sample base is not statistically valid. But it will give you more than a guess unless you have magnetic
plugs or magnets on your filter.
I have received comments from people who have used VW transaxles with 150 to 200 hp engines in
racing situations, where they say the synchronizers are expendable, but the gears are expensive and
hard to come by, so GL-5 is better. It will make for harder shifts, but if you are going to quadruple the
torque, you may be better off that way. It is a personal decision.
Here is a simple explanation of the problem from Nissan:
It is interesting to note in this explanation by Lubrizol how the new SAE J2360 (replaces the GL-5
rating) is explained:
"SAE J2360 is a global quality standard specified by many North American OEMs and
by growing numbers elsewhere in the world. The rigorous approval requirements,
including controlled field testing and independent committee review, ensure that products
approved under the SAE J2360 Standard meet the very highest demands of axles and
non-synchronized manual transmissions." (emphasis added).
The important part of that statement for us is the last part: "and non-synchronized transmissions",
since our transaxles are synchronized.
Here is a synchronizer that has been worn by GL-5 oil.
You will note that there are no longer any teeth on the
brass, completely worn or "peeled" away
The Difference between GL-4 and GL-5 gear oils 10 of 12
Here is a chart that shows the behavior of several transmission oils at start-up temperatures.
The Difference between GL-4 and GL-5 gear oils 11 of 12
Here are those same oils in normal operating temperatures once everything has warmed up.
Product possibilities
I hate to call these recommendations, but will list what I can of products that I know of and that are
available in the US. As always, this is as of this writing, and all information is gathered from the
Internet. Local availability will vary.
First lets remember that the details are in the spec sheets. Some brands make it hard to find the
information.
The Difference between GL-4 and GL-5 gear oils 12 of 12
I can applaud Mystic Oil's clarification when they start their spec sheet this way (although later in the
sheet they only mention the various GL classifications. Note how they say "and non-synchronized
manual transmissions."
Texaco, on the other hand, does not mention this
limitation, although it is the same. They just don't
mention transmissions at all in their 80W-90 gear
oil. They leave it up to the consumer to realize that
transmissions don't use normal gear oils (as such).
The list:
My list is a short one. If you want me to add something I can look at it if you send me a link. I've been
looking and don't see much. These are listed in alphabetical order.
• Amsoil Synthetic manual Transmission and Transaxle Gear Lube 75W-90 GL-4
• Citgo Citgear Standard XD 75W-90 GL-4
• Pennzoil Gearplus 80W-90 GL-4
• Pennzoil Synthetic 75W-90 GL-4 (from the spec sheet, the best overall viscosity curve)
• Quaker State Multi-purpose Gear Lubricant 80W-90 GL-4
• Redline MT90 is a good possibility, although their
web page has this disclaimer. Note my
observations on page 6.
• Shell Spirax G SAE 90 GL-4 (I do not recommend this due to it's extremely high low
temperature viscosity.)
• Shell Spirax GX 80W-90 GL-4 (I do not recommend this due to it's low high temperature
viscosity)
• Chevron had a product called Chevron Manual Transaxle oil that was excellent, but I cannot
find it except in clearance sites. I don't know if it has been replaced.
In general I would not use a 75W-90 in a Corvair transmission unless it was synthetic, but there are
some very good GL-4 Synthetic 75W-90 oils in the market for Mercedes, Volvo and Mack truck and
bus transmissions that would be acceptable, especially in colder climates. Look for one where the
viscosity at 100ºC is 15 cSt or above.
I hope this clears up some of the confusion and avoids transmission damage.
If you have additional questions, feel free to send them to me at oil@asboman.com
If you have not read the report on motor oils, you can find it here:
Selection of the right motor oil for flat tappet engines.
2007 Ford Edge SEL, Powerstop F/R Brake Kit, TXT LED 6000K Lo & Hi Beams, W16W LED Reverse Bulbs, 3BSpec 2.5w Map Lights, 5W Cree rear dome lights, 5W Cree cargo light, DTBL LED Taillights
If tuned: Take note of the strategy code as you return to stock (including 3 bar MAP to 2 bar MAP) -> take car in & get it serviced -> check strategy code when you get car back -> have tuner update your tune if the strategy code has changed -> reload tune -> ENJOY!
