I have used royal purple on my other vehicles because one of my car enthusiast friend raves about it. I have used both the regular ones and the new hps formula in my JDM vehicles. Both very disappointing and couldn't tell the difference at all. In fact, the oil sheared super fast and didn't seem to lubricate well after the first couple hundred miles. Initially, it did quiet down the engine, but then it became even worse than before, lifters have ticked so loud that i had the change it out. Also, it didn't seem to rev any more smooth or better. Lastly, I love the color purple, but dark purple motor oil is stupid. When you drain it you can't even tell if it is still clean or not. Overall my experience with RP is "meh". Won't recommend. But if it makes you happy, go for it!
I have been running Motul 300V 5w-40 in my DD e90 335i for a while and it is awesome! Lower engine temps, revs are smooth and very noticable when compared to OEM. Very stable temps during track days and UOA looks splendid. VERY LITTLE oil consumption vs OEM or other oils that i have tried in the n54!! HOWEVER, it is not meant for extended drain intervals. In fact, I drain and change my oil every 5k miles or so. Remember, it is still intended as a "race oil" to begin with, even with all the detergent and additives package i am not risking having it in the engine for too long. Again, is it overkill at especially when it's $17/quart? Ya, i think so. But I love the thing, smells SOOO good too
(not that it matters)
In my parents E-Class Merc they have been using liqui moly and it seems to be fine. Less oil consumption, revs silky smooth but not as noticable as the Motul in the 335i. Maybe because of blowers? Can't comment much except it is a quality product that does a fine job.
If you want ultimate VI, then look no further than ENEOS, in particular the ENEOS Sustina. If you have not heard about them that's because they are not popular in EURO cars. But they do work great. Many good reviews on JDM forums and I have tried it on my other vehicle as well. Great stuff. Does it make more power and protect better? Power - i don't care, protect and clean - i'd say yes. I have ran it in my Honda S2000 and every drain (besides the first two) comes out clean. Clean as in it still looks orangy yellowish golden colored like when i first poured it in!!!
Redline, they are ester based like the Motul, but i have never used it sadly. But i believe that for sure it is an incredible product. I have read many threads and reviews and Redline always seems to have excellent feedback.
Lastly, this is not about oil, but rather an oil additive. (This is when i put my flame suit on...) OIL ADDITIVE, you've read about it, and you've heard about it, and to most people's consensus it is a big NO NO. That is before I stumble across an engine oil optimizer by the company RS-R from Japan. The product itself is called RS-R Ran-Up. Do yourself a favor and google it. Read some forums regarding this product. But from my experience this product is AWESOME. It is a synthetic powder-based lubricant that serves as microscopic ball bearings on your engine parts to reduce friction. I was very very VERY skeptical before but I now use it in all my E90 335i, E39 M5, and my NSX. Revs are noticably smoother, Revs build a slightly little bit faster, engine noise decreased due to reduction in friction, even shifting smoothness is better. Placebo? No... I tested this out on my parent's Van and my girlfriend's car without telling them. They felt the difference and asked what I did =D RS-R claim that it also makes more power, but i don't care again.
For anyone who is interested and is in the SOCAL area, give them a call because they had been giving curious customer a FREE sample and 2 FREE dynos at their facility to let you see the difference and to try out this awesome product. Don't know if they are still doing that though. But once you do try this thing, you'll be hooked, for sure. And this addiction is crazy... the RS-R Ran-Up retails $50 a pop. In a larger engine like my E39 M5, it takes two bottles...
Good Luck, have fun, research up, and use what you like!!