Truetrac diff oil

[villn808]

The oil company will match their oil to the manufacturer's specifications. The manufacturer has done testing to determine the best oil for use.
I'd stick with those companies that publish and stand by their own recommendation. At least you can go back and tell them they made an error, and make a claim for a new/repaired diff.

Your diff builder is not liable for providing an incorrect suggestion.

This looks like an option,

https://www.penriteoil.com.au/assets/pis_pdfs/PRO GEAR 85W-110 (Mineral.).pdf

GL5 and can be used to replace both the 80w90 and 140, stand up to a little more heat than the 80w90..?

 

[Immortality]

Diff builder probably has years of experience under his belt fixing fucked diffs. Depending on how much power you put through the diff, gear ratio and how you drive it can have a big influence. Although 80w-90 is probably fine for stock power driven as the car was designed it may be found lacking when you are baking the tyres off the rear end. Excessive heat will break the oil down. There is a reason modern cars with big power run diff oil coolers.

So it comes down to who's warranty you want. If you run something other than the diff builder recommends and the diff fries sometime in the future he'll simply wash his hands off the problem. Harrop may or may not warranty the centre but then you are still out labour to fix it.