Best Lubricants

I've always used Redline MT 90 for transmission (GL4), Redline 75W90 (GL5) for gear boxes. Works fine and seems well reviewed, so I've gone with it. I did recently start using Lucas 75W90 for the gear boxes, just because it comes in a gallon jug which is nicer for me than a ton of the little quart jugs the Redline comes in.

 

DOT 5 is silicone based, but I've seen plenty of guys use it in old cars for years without any issues. It's a little different in how hard it is to bleed, but it should be fine to use in our trucks. Silicone doesn't damage rubber seals.