Check out Rotella. I used to buy it in 5 gallon pails at
farm/fleet stores or at truck stop service centers at unheard of
low prices per quart and used it in my cars, motorcycles and
lawnmower. Although it's primarily marketed as a diesel oil
it also carries an API gasoline rating and works very well
in gas engines. They have recently changed their formulas
but I'll bet you can find one that fits your needs.
I got turned on to the T-syn 5-40 by the motorcycle
community years ago..I figured if it works well in
150HP, 1200cc high compression engines with shared
gear boxes it'll work well in my Infinity, Goldwing
Mercury, Honda, and lawnmower as well...and it has.
Good luck