just a quick note as ive been servicing my vehicles and changing all lubes in my trucks this week. In the transmission (sm420) I have used 90wt gear lube (gl4/gl5), 85-140 gear lube, and have even used lucas gear oil additive in the past. This time i switched to 90wt mineral oil from NAPA as suggested by many in this forum as well as original lube recommendations. After switching it was hard to believe the transmission was the same one as it shifted smooth as butter both up and down ... ill be changing lube in my other trucks as they come due for change
