Personally, I'd save FOR a GDI cleaning service BG or Walnut etc (see this
discussion), and not a transmission flush. The transmissions are pretty sturdy, but the PTU is another story.
You can approximate a transmission flush by draining the fluid multiple times through the drain plug.
- Before starting this process, get the transmission up to operating temp with a good run 10-20 miles non-city, then park it.
- With your foot on the brake, run through the gears each time before starting the drain process.
- Drain the transmission fluid into a graduated pan.
- Fill new transmission fluid (same quantity as drained).
- Start the engine, let it run for 3 minutes.
- Repeat from step 2 until in total you have put in 15 new quarts or more.
Once you have equaled or exceeded the stated transmission fluid capacity with the drain & fills, you should be good. The manual says 10 quarts, but probably does not include the fluid held in the torque converter, not sure on what that amount is. On the Edge (6F50), a total of 15 new quarts is recommended with this procedure, so I would think the same with the SHO (6F55).
Some people even set up a container of new fluid with tubing that goes into the fill tube (sealed of course) to gravity/vacuum draw new fluid into the transmission as the old fluid drains. If you haven't done this before, avoid it for now, to be on the safe side.
You can tap into the cooler lines and do a flush yourself that way, but I don't recommend that for folks without significant experience in automotive servicing.
