One of the car mags had a test where they replace all the oils with I think royal purple? Not sure if thats right. They did a before and after dyno and gained 10hp. This was the engine, tranny and rearend oil. It also cost them over a 100 bucks to do that.

I have a 85 chevy that I have used every aftermarket additive there is, starting at 130,000 miles and it still runs good now even though it is over 230,000 miles

My first 455 droped 15 deg when I added, dare I say it, Slick 50.

I can also see why you wouldn't want to use it in a rear end. The heavy weight oil is suppose to stick to the gears. The syth oil, I would think, would be more apt to slide off?

One last thought. Some where on the Classical pontiact site there was a link to a oil test on a fleet of taxi cabs were they test a few diffrent brands, and changed some at 3000 mile and others at 6000 mile. When they tore the engines apart there was no diffrence in ware between them. Don't remeber if they test any synthetics in that test.