16% is on the performance side, 20 is on the longevity side. I usually get 18% if i'm not racing because it's in between, plus it is a common percentage. but i wouldn't get too worked up about the longevity thing. It won't dramatically effect your engine's life, and you'd want to get a new engine every 2 years or so anyways. If your not racing, go ahead and get 20%, you probably won't see a difference in the performance.