the motorcraft 75w85 that the manual calls for just seems too thin and I am out of warranty.

truck is a 2015 and the rear is the super 8.8. Mostly use it for towing.

Pinion seal is leaking and it’s time to change my front and rear diff oil again and put in some new gears and stuff.

 

what evidence makes you think that factory spec is too thin for our application?

 

I'd rather the gears in the diff swim through the thin, specified oil than through a thick soup. Same for the engine for that matter.

 

If you want to go a little thicker 80w90 wouldn’t hurt anything, maybe a fraction of a MPG

 

I would switch over to 75w90 synthetic gear lube for a little extra margin. w90 viscosity is w40+ in the SAE motor oil scale vs. w30 for w85.

 

This is what I did.. Gen 12 F150 ran this weight.

 

75w140 will work perfectly fine, it’s what they used for years. Only reason they switched to 75w85 is for a very small fuel economy gain. The thicker oil will protect better... especially when towing.

if you wanted something in the middle, amsoil makes a 75w110.

 

Yep I run royal purple 75-140 good stuff

 

I always use Royal Purple in the limited slip differential on my F250. It doesn't require a separate additive that most off the shelf greases need for the limited slip.

 

Good thread. I guess if one were to really think about this...like why did they go to a lighter weight. It is true that the lighter weight lubricates very well. But if it is truly because it helps in the overall MPG rating, does heavier make sense? The other question would be, what is the failure rate of rear diffs in the life span of the vehicle with OEM recommendations?