I'm probably going to open a whole can of worms here. I went to Valvoline's HQ here in Lex this past summer. I saw piles of test data on Valvoline brands, Mercury brands, and about every other brand you can name. Amsoil, Pennzoil, you name it. I was asked not to publicly discuss who makes Mercury oil, but now that I know, if I had a opti I wouldn't run Merc oil in it. Now, don't get me wrong, i've never been a Valvoline fan but, after seeing the tests, I'd go with Valvoline and then Amsoil 2nd. Valvoline also has a biodegradable synthetic on a limited basis. If you want to know more about what I'm talking about on here, call John Webb of Marineworks at 859-509-6660. He is tight with someone at valvoline and knows far more about it than me.