Regarding a 2001 F-250 SD, rear differential gear lube...

I replaced my dif cover. I saved, filtered, and reused the gear lube that was in the dif, 75W 140 synthetic. The lube only had 15k on it.

The new dif allows for more gear lube. Can I add a quart of 75W90 to the existing 75W140 without any problems?

 

Probably. I wouldn't.
Hired Gun thinking...........you put used oil back into a $30,000 truck???
If it was mine I would have put brand new Mobil-1 synthetic gear oil back in there. A tank of gas would have cost more than that.

 

I know that stuff is expensive but you should use new. Don't mix used and new together to save a buck.

 

Yeah reusing used gear lube is penny wise, dollar foolish. Probably OK, but why? I could see using it on a real beater but...

Oh well. You can mix different viscosities but it is better to use the same weight and brand. Also if you have a limited slip, you may need to add limited slip additive.

Oil is cheap, use brand new at all times. Figure 15K on the last batch made it a nice differential flush.

Jim Henderson