"If the RP is designed to have a higher film strength then it very well may be just fine for your transmission. I work in the gearing industry and you can see large gains from changing oil specs. I'll admit I'm no expert, but if RP recommends it I'd be fairly secure that it'll work alright. If you are very unsure, use a magnetic drain plug, or drain the oil out in a few thousand miles and run a magnet through it. If you see metallic particles there is obviously wear going on. Keep in mind they may be remainders from the first oil change that weren't flushed out initially though."
To be honest, RP is one of my least favorite of the large oil brands. For years, they have been heavy on the gimmicky marketing and puffery, but short on real innovation. Currently they are a sponsor of the Iron Man movie ... WTF does that have to do with automotive lubricants? Are we to believe that Iron Man can do all the things he can in the movie because Royal Purple works so well?
I just don't have a lot of respect for their claims of higher film strength. Most Royal Purple formulas are a mix of petroleum/mineral oil and PAO ... which is what most companies use these days. Talk of "higher film strength" sounds like they are comparing their current formula to mineral oils from decades ago.
Here is a thread on BITOG where we discuss one person's experience using Max Gear in their car tranny (yes, I participated). He sent a sample of his used oil off to a lab to see how it fared:
http://www.bobistheoilguy.com/forums...93#Post1037993
It's best not to trust a visualization of metallic particles in used oil ... just not scientific enough ... and once the tranny is showing that kind of wear, most of the damage is done.
If you choose to run the lighter oil, fine. But I'd have a sample tested by a lab after 10-20,000 miles just to see how it's holding up.