Hi Dave -
There have been those who voice concern over using some of the synthetics in the MN6, the concern being that some lubricants can cause premature wear of the organic synchronizer friction materials.
I have read that Royal Purple is ok to use, and I would not be afraid to use redline either, just make sure it is for manual transmissions.
Myself, I will stick to the Dexron - III factory fill lube. It's cheap and I can change it as often as I fell necessary. 4.1 quarts are specificed as the fill quantity in the GM service manual.
To drain/refill:
There is a drain plug (with the word DRAIN cast in relief on the "passenger" side of the trans) just forward of the halfshaft, near the bottom. Remove it and allow the fluid to drain.
Replace the drain plug.
On the other side of the trans is a plug with the word FILL cast in relief on the trans case just above it - remove that plug.
Pump 4.1 quarts of Dexron - III into the trans (I use a small fluid pump I bought at AutoZone just for this purpose for about $10).
Replace the fill plug.
Put two ice cubes in a small glass, fill with 1.5 oz of Jack Daniels.
Toast to your success.
best regards -
mqqn