bought some valvoline 75w90 gear oil for the w51 tranny, and i noticed it says limited slip on the bottle? What is that mean and can I use it in the tranny?

 

Does it say GL-5 on the container? A "limited slip" rear axle gearset takes a special additive in the gear oil to function properly. "Positraction" "SureTrak" etc some names the manufactureres used. GL-5 has the additive. GL-4 does not. In the past, the additive contained in GL-5 was alleded to attack yellow metals, copper, brass, bronze, etc,.Synchromesh transmissions contain some parts made of bronze.

I think that it is no longer the case, that GL-5 could harm a standard transmission. I used Sta-Lube GL-5 in the 4-speed transmission of my '65 Chevy from 1975 on for many thousands of miles and noticed no problem with the shifting..

You can use any lubricant you want, but your best bet is to use the recommended type specified in your owners manual. My '89 pickup manual specifies type GL-4.

 

The dual spec GL4/GL5 Valvoline 75w90 gear oil will not harm yellow metals. It has been proven to be rather mild. It will work. For the smoothest shifting possible you want a GL4 made specifixally for manual transmissions in mind, with the correct friction modifiers for the synchros. Cheap option is Stalube, best option is Redline MT90.

But that valvoline will still work fine.