Just use the Mobil One and keep it changed when the OL says to change it and you will be fine. There is no need to add anything to it if you service the car properly.

I have used Royal Purple in other cars in the past but went back to Mobil when I got the SS as it was what GM recommends and I wanted to keep the warranty active. Royal Purple at that time was not a GM Approved oil. It is still a good oil but they never paid to get the GM rating.

Cost was the same for me on either. They may have changed their rating since then but I have not checked it lately. Note we sell it where I work and that is how I got a similar price.