I have a TKO 600 with no external coolers/pumps, etc. It has about 4000 miles total on it - weekend/sunny day car only. I have run it pretty hard on the track a handful of times (maybe 6 days over the last 3 years). After the initial break-in, I changed the oil to Valvoline MTF. The manual calls for ATF for the 1st 500 miles, then GM Synchromesh and Valvoline was what O'Reilly had. The bottle says it is compatible with the GM stuff. Over the last few weeks my tranny has been hard to get into gear. Any gear. (I know, I need to check the clutch disengagement). Anyway, I decided to change the oil. The tranny is known for its notchy shifting and I have a buddy who raves about Amsoil, so I figured I'd try it. When I drained the old Valvoline stuff out, it looked like chocolate milk. It was still pretty thick and I didn't see much particulate material (I didn't look that closely at the time). It didn't smell burnt. I don't have any other fluids that can get in there. No water/antifreeze/motor oil, etc has access to the tranny. It is just a self contained unit. It has not been out in the rain - no way for water to get inside. Would the residual ATF left in the case after draining, mixed with Valvoline MTF cause a chocolate milk appearance? I haven't put the Amsoil MTF in yet. I hate to "waste" $20/qt Amsoil if I will need to drain and fill again in xxx miles. Should I fill it with cheaper MTF or other gear lube, run it for 100 miles, then D/F with Amsoil? Any other thoughts?