I run Amsoil 20w-50 racing serve synthetic in mine. With 20 hr change on motor and drive.

Well there ya go, thats the reason he SHOULD go synthetic.

By premium, I just mean any name brand, not the Royal Purple, Amsoil or the like. Valvoline, Penzoil, etc.

here are the recomandations from the merc 500efi service manual

I only use Penzoil 50w..... but of course thats suggested because the motor is built. I've used the 25-50 from Penzoil, not sure if its because of my motor clearences or not, 50w seems to not heat up as quickly and runs cooler. Probably not suggested for a stock 500EFI? not positive though....

oil is for pu$$ys, i piss in my motors.... i get 105 out that bit ch...

I wonder if they take inconsideration some of these engines are up north in real cold water ????

I guess if ya change it often enough ,before fuel dilution,or when its breaks down from high temps.I been running the Valv 40 for 16 years in different boat blower motors and have never had a bearing problem or oil related problem

I am sure that if I ran the sythentic lighter oil it might make 5 more HP but so would 10 w mineral base oil