Hmm. With 50,000 or more many people will tell you that it's a bad idea to switch since the engine is "used" to running with conventionals, assuming that's the case of course... Synthetics are always better oils than conventionals at cooling and lubing, and Royal Purple is for sure one of the best if not the best you can get.

I personally don't think it's a bad idea to switch since I had no problems when I switched to synthetics in my D16Z6 that had over 200,000 miles on it. And I don't think it will hurt anything to use the gear oil, but you should keep checking to be sure. It's more likely that it'll be advantagous.