I've used Castrol 20/50 in the engine and the gearbox. I'll add a bit of Seafoam each time I add a quart, and put a little bit of Seafoam in the gearbox, too. I blow through oil at a fairly prodigious rate -- usually about 1 quart every 250 miles, and probably more through leakage than burning. I know I'm due for a rebuild, but I'm trying to put it off as long as possible.