Are there any mods on the car? How is the oil consumption? A lot of guy w/RSX-S burn oil especially with Mobil 1. IF you are getting a good result with AMSoil, that's great but you should be runnig it out to at least 6000 miles to get some use out of it. Myself, in this type of engine/car I like MaxLife or Pennzoil 30 weight. Synthetic 30 weight just does not appeal to me since it's such an easy viscosity to formulate for, that is you don't need synth base to make it and the $10-$12 a gallon jugs of dino oil out there work great in the sub-10k miles Honda application.
"Best oil" is kind of a wierd concept in itself. Define "best" and go from there, imo.
If I *was* paying full price for synth, either use it for a good long time or more appropriately use it for a slightly thicker viscosity it offers in oil like Mobil 1 0w-40, German Syntec 0w-30, MaxLife Synth 30, Mobil 1 10w-30 Hi-Miles.
If you are burning oil like some of the RSX guys I knew, AMSoil 5w-30 is not really the way to go. I put a bunch of Mobil 1 burners on MaxLife 30 and 40 and it solved that issue for them. It's hard to beat AMSoil, but slightly thicker oil and dino/HM in general just seem to work better.