I just switched to redline, 5w20. I was running Mobil 1 Synthetic because we used it in our competition race kart and would rev that little 5hp Briggs motor to 9k and never had any issues with motors. I figured that said enough. Switched to redline only because it was recommended by Kenne Bell. They stated in there testing they ran cooler and gained 15hp by switching all fluids to redline. I did not see the actual dyno comparisons but Kenne Bell does a ton of research and development and has proof to back all claims.