For what it's worth, here's an experience I had with synthetic gear oil. I had a 68 Sweptline with 4.88's in the diff's. I was also running 38" tall tires, fresh non-synthetic gear oil and driving at a pretty high cruise speed of 75 mph. I drove to a destination about 4 hours from my house. The rear end was radiating so much heat I could not get my hand withing 2 feet of the diff cover., because it was so hot.

That high heat concerned me. When I got back home, I swapped in Redline synthetic gear oil, and made the same trip a few weeks later. After 4 hours on the freeway at 75mph, I was able to put my hand directly on the warm diff cover. The synthetic gear oil had made an amazing difference.