HummBebe, go to their website and watch the videos, and read the articles. I use Royal Purple in everything I own, including my chainsaws and industrial sized weedwhackers. I've got it in my Dmax, my tractor, and even my quad. It's really really good stuff and it's dyno proven to make the difference (Hot Rod magazine did the testing, the videos on their website)
http://www.synerlec.com/video/video.html
I started using it on my own because I use it in our equipment at work. (I'm the lube guru) It was a real eye opener to see it drop one of our compressor's operating temperatures 25 degrees. It's extended the run times on our 4kv feedwater pump motors to ridiculously long intervals. It's the real deal and well worth the price. I get mine shipped from this guy:
http://www.karlracing.com/html/royal_purple.html
Another way RP helps is that you have a knock sensor that affects the engine's output. RP products (and it's Purple Ice coolant additive) lets the engine run cooler at max load. Again, I've seen the cooler operating temperatures on power plant equipment where I have a lot more monitoring devices installed so I believe to be the same in their engine lubricants also. Yeah, there's a thermostat, but before I started using the RP in my Hummer, it did move above the where the thermostat usually keeps it, on RP, it doesn't.
If you have ANY questions about what product to use where, the guys at Royal Purple's customer line will help in a minute. 888-382-6300.
You'll hear or read many people saying it's a waste of money and that it's not worth it. Bull, dynos don't lie. People spend hundreds of dollars on exhausts, intakes, filters, chips, you name it, why not spend a little extra and give that I5 some extra lovin'? The added benefit of protecting your engine that much better is another benefit. As long as the oil change intervals are, the cost of RP is inconsequential, particularly considering the aforementioned benefits and improvements.