Just finished DOI skid plate prototype

[f5fstop]

Didn't know you were talking a manual, didn't read the entire thread, but answered your email thinking auto.
The manual uses a manual trans fluid, non-synthetic. The auto trans and transfer case uses Dexron IV and is also non-synthetic. Axles use synthetic fluid.
With a manual, you might be ok using synthetic, never seen anything against its use. For an auto, I don't recommend synthetic; too many people I know who had Vettes had problems with this trans and sythetic fluid. Problem with some synthetic in a trans is that trans fluid is rough. It feels slippery as hell, but it has to aid in friction, so it is finely balanced between being slippery and create friction.
So, I would guess for a manual, just drain and fill every 25K and you should be ok, or switch to a synthetic that can take the heat better.

You are correct, manuals do not increase the heat of the fluid like autos. There are no pressure passages with fluid pushing the countereffect of springs, pressure to apply drums, etc,. and the torque converter which causes a lot of heat.