Tried: RP 20/50, Amsoil 20/50, & Mobil. BBC, all roller, 12.5 comp. pistons. I could not tell you what the difference was between them in quality, power, oil pressure, & longivity, etc... They all worked fine... I change it often & run synthetic because it is a hell of a process how they break it down over & over... truth is, the Amsoil rep here told me that all synthectic oils ingredients are the same & come from the same place. Some have higher levels & different mixtures... but the ingredients are the SAME. When I change it, it's the same going in, & the same comming out. I personally do not believe in a "miracle oil" that has patented specail powers to make a motor do things it's not already made to do. I am no expert either... but I have studied oil & the manufacturing process some, there are no cosmic secrets that oil can reveal to make a motor more efficent. For protection, it is a fact the synthetic oil has better sheer protection & does not break down as fast as petrolium. Having said this, why but a blend of the 2???? Good luck!!