Horsepower TV just did a show where they changed out rear end lube, tranny fluid and oil all using Royal purple stuff. They dynoed before and after and gained 10 HP just by changing all the fluids to this to reduce friction!
The same test (I think) is also in the latest HOT ROD magazine. They took this guys stroker smallblock Ford (fairlane?) and like the other poster said, added to crankcase, trans, and rear, and freed up a nice chunk of power. Pretty informative article, too. I want to put it in mine once I get the oil burn taken care of.