I checked the Chilton s-10 manual and the Motor lube service guide, both say to use atf, exept for the '87 Camaro, which calls for gear lube. I can't explain why the '87 would be different.
I can say that I've seen what happens when you don't use the reccomended lube in a trans. We had a kid who had a '91 4x4 dually dump truck. It was equipped with tha NV4500 5 speed. This trans calls for 50 weight Castrol synthetic oil. There's even a tag on the pto cover that says this, and give the Castrol and GM part numbers for it. This guy was insistant on using 90 wt. The last I heard he was on the 4th tranny because the kept wearing out. I drove the truck once, after the 3rd trans. The syncros were so sloppy that I could slide it in to gear without using the clutch. This was at idle, stopped.
On that note, I would think if there was a "sloppy" feel to the shifting with the correct fluid, there are probly worn parts that need to be taken care of. Using heavier oil is only masking the problem. Kind of like putting 50wt in the engine to quiet noisy rod bearings. I would suggest following the manufacturer's suggestion for the original application.
