Out of Grade Diff Oil

[EddieCairns]

If I get a used oil analysis of a diff oil, grade 75W/85 and the oil has gone down to the upper area of an 75W/80 oil, does that mean the oil is no longer fit for purpose, I assume not as most diffs have long asumed lifes like say 6 years?

I will be asking for a TAN in both the virgin sample and the 10,000 mile sample. What level of TAN would cause concern?

 

[userfriendly]

Too many what ifs.

 

[MolaKule]

If I get a used oil analysis of a diff oil, grade 75W/85 and the oil has gone down to the upper area of an 75W/80 oil, does that mean the oil is no longer fit for purpose, I assume not as most diffs have long asumed lifes like say 6 years?

I will be asking for a TAN in both the virgin sample and the 10,000 mile sample. What level of TAN would cause concern?

Oil degrades due to oxidation thickening and mechanical shearing (viscosity reduction).

Change diffy lube once a year and you'll be able to sleep well.

 

[Rygrego]

If I get a used oil analysis of a diff oil, grade 75W/85 and the oil has gone down to the upper area of an 75W/80 oil, does that mean the oil is no longer fit for purpose, I assume not as most diffs have long asumed lifes like say 6 years?

I will be asking for a TAN in both the virgin sample and the 10,000 mile sample. What level of TAN would cause concern?

Diff oil does not change that much because it doesn't experience the byproducts of combustion. Shy of water getting in the diff a quality gear oil will last quite a few years.

 

[johnmyster]

Folks on this forum get all bent out of shape when a UOA shows that a barely Xw-30 that "fits the spec" falls into the top of Xw-20 range after 5000 miles. It's like they think the manufacturer only did abusive endurance testing with perfect lubricants that stayed 100% for the testing cycle.

If the manufacturer specified 75w-85 and gave it an interval, they knew full well it wasn't going to be new oil forever. If it bothers you, use a 75w-90 synthetic.

 

[OLEJOE]

When in doubt, change to Severe Gear 75w-90.