Two questions about gear lube

[wavinwayne]

1. Would Valvoline 85w140 semi-synthetic gear lube be less likely to leak past seals & gaskets than Motorcraft 75w140 full synthetic gear lube? Why?

2. Can I run Valvoline 85W140 semi-synthetic gear lube in a differential that specs 75w140 synthetic? Why?

 

[Pablo]

1) Maybe. But if you have a leak, you have a leak. I suppose the 75W-140 should flow better and may flow out of an existing leak faster. But in all reality it depends on the formula, the flow, the seal conditioners....so it's difficult to say. Is the MC oil a PAO/Ester? The Valvoline is?

2) Probably, but if your car calls for synthetic 75W-140 and takes maybe 2 qts or so, why mess with it? Some diffs get VERY hot, a "semi-synthetic" that is 85% GroupI,II....and small amount of synthetic may not be happy after a hard run.

 

[wavinwayne]

Pablo,

FWIW, the vehicle in question is the wife's Expy. It serves as a grocery getter & kid hauler. No severe duty or hard runs.