I ran Brad Penn until I road raced and changed over to Red Line due to it being a full synthetic. Either are great until you get into the 250 range in oil temp then the full syn is key. My engine builder has experience with engines run and inspected with both. They are both capable of little wear.

Thanks Greg.


Greg

I've heard this before and I'm guessing... Because of the clutches in a Posi unit? WE run Syn with our spooled rears and never an issue

Not many clutches in a true trac. :unibrow:

Good point, I never delt with those. Way back when I was young... It was beefed up GM or Moroso posi units.. Tells you how long it's been since I've had a street car :cheers:

For any application that includes a combination of gears and frictions (like a diff), it's best to call your favorite manufacturer or dealer for a recommendation. They'll have a database of hardcore customers who have tested differing levels of friction modifiers cross every possible combination of parts.

Or if you're the shy type, follow the spec of a new ZR1 or similar. It's probably synthetic, but has just the right amount of friction modifier for combined street / track day use.

Cut and pasted straight from Detroit Truetrac website FAQ's:

Detroit Locker

What kind of oil should I use? Can I use synthetic? Do I need friction additive/modifier?
A quality petroleum/mineral based oil works best in the Detroit Locker units. We do not recommend synthetic oil. Friction additive/modifier is not required.

Here's something that might surprise a few people --- it did me!




Viscosity measurements following the KRL Shear Stability Test revealed that seven gear lubes sheared down below the minimum viscosity requirements (orange), failing the shear stability requirements of the SAE J306. The two gear lubes with the largest viscosity loss, as reflected in the following graph, were Royal Purple, losing 40.6% of its viscosity, and Torco SGO Synthetic, losing 35.2% of its viscosity. Royal Purple was the only gear lube to fail both the initial viscosity require- ments and the shear stability requirements. It started out too thick and ended up too thin. Torco SGO Synthetic, which had the highest VI in the previous graph, finished the shear stability test as the thinnest of all the oils at 9.97 cSt, far below the minimum 13.5 cSt requirement. Lucas 75/90 Synthetic, with an initial viscosity that exceeded the maximum requirements by 20.8%, passed the shear stability test, but lost 34.5% of its viscosity, the third largest loss of viscosity. Both OEM gear lubes, GM and Mopar, failed the minimum viscosity requirements after the shear test. Of all the gear lubes tested, half did not meet the SAE J306 shear stability requirements.
AMSOIL Severe Gear 75W-90 was in the proper initial viscosity range and retained the highest viscosity after the shear test with a viscosity of 16.03 cSt – the mid-point of the SAE 90 viscosity grade.

Read the interesting statements made in the first paragraph of this "white paper" on gear lubes.... Very interesting statements about low floorboards restricting air - and the close exhaust systems to the diff... all things that PT cars would be very guilty of!



http://www.oilteksolutions.com/GearLubeWhitePaper.pdf