I know this is an old thread, but thought I'd share my perspective on the gear oil question. My take when it comes to any kind of oil, including gear oil, is to only use oils that are CERTIFIED to meet a particular specification. This does not mean that an oil made by Amsoil, Redline or any other boutique producer is bad, but it does mean that as a consumer it is very difficult to determine if the boutique oil in fact meets all the criteria for an oil specification such as API GL-5/MT-1/MIL-PRF 2105E. I have also learned that some oil specifications are more or less a joke. For example, when it comes to engine oils, the API SN or SN+ specifications focus mostly on fuel economy standards and are not competitive with most European spec oils. Lubrizol provides a really great spider chart that allows us to see the difference in performance across a wide range of motor oil attributes. If you compare the API specs versus others, you can see what I'm talking about here:
https://www.lubrizol.com/Lubricant-and-Fuel-Additives/Engine-Oil-Additives/ACEA/Relative-Performance-ToolBack to gear oil for our vehicles. Given the choice, I prefer to use a fluid such as Mobil 1 Delvac gear oil. This product is formulated for heavy duty use in big rig trucks and is intended to have a very long service life (500,000 miles or more). It can be found by special order from a few sites on the internet (maybe even Amazon now) in the 75w-90 weight. It is a full synthetic and meets some very stringent specifications and approvals from a wide range of truck manufacturers. See here:
https://www.mobil.com/en/mobil-delvac/products/gear-oils/mobil-delvac-1-gear-oilI recommend this lubricant for the rear differential since it's the right weight to use (slightly thinner cold viscosity than the OEM spec which is a good thing) and the rear diff is somewhat challenging to get to for fluid drain/fill.
The PTU is a more challenging scenario. Ford requires a 75-140 weight fluid. The OEM fluid has a tendency to turn to sludge when not maintained, and I can't recommend it for that reason. As an aside, it appears Ford is very proud of this product, they use it widely across their product line. Also, ConocoPhilips appears to be Ford's partner for developing fluids. If you try to find non Motorcraft fluid that explicitly state they meet a Ford spec, often times only ConocoPhilips will have one available when no other producer does. I am almost certain that this product is the same (or very similar to) the Motorcraft 75w-140:
https://www.phillips66lubricants.com/product/syngear-ss/.
For the fun of it, I did once try Amsoil 75w-140 in my PTU and I was not happy with the performance - I noticed bad odors and it also seemed like there was more drive train drag with it in there. The Delvac 80-140 is an option, but it only comes in large drums, which is not really feasible to obtain for me. I have not used it. The Mobil 1 (not Delvac) 75w-140 is a quality product which I have used with success, but it also has a friction modifier in it from the factory for limited slip differentials (not required in a PTU) which is not optimal. Schaeffer's and Motul both make a few interesting 75-140 gear oils, I have not tried them, but these could be good choices too. I'm not really a fan of the Valvoline 75w-140 based on personal experience, it tends to break down fast. I've been running the Mobil 1 75-140 for some time, it seems to hold up well and does a good job. I have not tried Redline or Royal Purple. I know a lot of folks love these products and recommend them, I worry about these products not being certified. I know you may be thinking, "well this guy used the Amsoil gear oil so what gives?" The Amsoil gear oil is not certified and I regret having used it as my experience was not good.
So, my conclusion is that given what is readily available, I think the best choice for the PTU is actually the Mobil 1 75w-140 and the for the rear differential the Mobil Delvac 1 75w-90. And, as has been stated many times before, swap the PTU fluid regularly . . .
I hope this helps.