Hi Michael

One of Eaton's websites states the following:

 https://www.eaton.com/us/en-us/products/differentials-traction-control/aftermarket-differentials/detroit-truetrac-faqs.html

"What kind of oil should I use for my Detroit Truetrac? Can I use synthetic? Do I need friction additive/modifier?
A quality, petroleum-mineral based, GL5 rated 80w-90 gear oil should be used.
Synthetic oils are not recommended for Truetrac applications. While some customers have found that synthetic oils works well in this application, Eaton has not confirmed under test conditions that all synthetic oils are compatible with Detroit Truetrac applications.
Friction modifiers should not be used because they will decrease performance (reduce differential bias)."

On this basis you should consider HD GEAR OIL 80W-90 (Mineral) - https://www.penriteoil.com.au/products/hd-gear-oil-80w-90-mineral . Having said this we have had anecdotal comments from customers indicating that, in their applications at least, the use of Gear Oil 80W90 which contains friction modifiers has been successful. So things are not always a straight forward as you might hope. Also note the comment about synthetics which was added more recently.

If competition is a primary focus your best course of action is to follow Eaton's requirements and use HDGO 80W90. The issue is that the minimum pack size is 20L.

Cheers

Costa

Penrite