Synthetic gear lube
 

Heat kills, so the problem with transmissions and rear ends is check the temp and if it needs coolling, you can use synthetic lube for some additional heat capabilities if recommended, or fit a cooler!!

I run an 8.8 IRS with Eaton posi with carbon clutches, Eaton recommend 80/90w mineral oil with the Ford/GM additive "NOT SYNTHETIC" the same thing would apply for the Ford LSD, Jaguar Thornton power lock and original type Cobra diff. I wouldnt take any notice of reps from oil suppliers saying their synthetic product can be used especially in LSD units, they mostly dont know, and I see guys here get their hotrod Cobra's up and running the first thing they do is put some Redline etc fully synthetic in the LSD..............!!!!!!

Toploaders, the race guys here have used ATF fluid for racing to make easier shifting (my road car still has ATF for 440,000K's) and no problems. With my Jerico they recommend any good quality synthetic 75/90w oil, but it hasnt got syncromesh which maybe affected by synthetic.

Synthetic isnt good for every application, some engine builders dont like full synthetics, and often recommend semi-syn or mineral oil. I used to think because more money is spent on synthetic lube I would leave it in my vehicle longer, to get my moneys worth, but it is probably better to be changed more often to get rid of any contaminants in the oil.

I used to work on the North Slope at Prudhoe Bay. Got down to -60F in winter. We would receive new pickups in the fall and ALL lubes were changed to synthetic before they were given to us. If the wheel bearings or rear diff were not changed to synthetic, the wheels would not move after sitting all night in low temps. They even used synthetic Auto Trans fluid.

These were a mix of Fords and Chev's and if it wasn't advisable to use synthetics, I believe they would have found out quickly.

I don't know where these tales come from, but I trust my experience, not unsubstantiated tales.

Our Cobra's sometimes have fancy transmissions or rear diff's that may change that. The best advise should come from the manufacturer.

Of course, you should always break a new engine in with dino oil, valvoline racing is what I use. After a couple thousand miles I go with synthetic and Amsoil is all I use.

We run the Royal Purpal 90 170 in race trim for the rear 9 inch diff no problems.we also run a standard lube for the street