i have been running Amsoil for about 12,000 miles and ran royal purple once before that. The first 20,000 was Delo400. My question is it really better? Is it worth it? I'm spending big bucks on oil and still changing at 4,000-6,000 because I worry about soot levels in the oil. Is it better to use conventional and just change more often? I just want to what is best.