I've looked around for any oil analysis results for Valvoline SynPower, but can't find anything on them. I know it is a group III oil,I was just curious on how it would do on extended drain interval (~5k miles). I think I may switch to this oil, since after very much contemplating, I've decided that I can't afford oils by Amsoil, schaeffers, or Redline. (College makes me poor). Yes, I do know that I get free shipping when I order $250 worth from Schaeffers, but I don't have the $$ right now. So, I was thinking of either using Valvoline synthetic and changing it every 5-6k miles or use Castrol GTX and change it every 3k miles. I've gathered that Castrol GTX shows pretty good results on anyalysis sheets, but I have not seen much of anything for Valvoline's Synpower. Are the benefits of Valvolines synthetic that much better over Catrol GTX?