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.