30W and change once a year if you don't drive much. You also don't need a lot additives in the oil for a stock engine. The valve springs have very little pressure even when open and the cam has really mild ramps with little lift. Run your favorite brand oil you like, just stay away from the 50w's, you don't need the extra stress the heavy oil puts on the oiling system. I run standard 10-w 20 in my 250, no additives or race oil, going on 10,000 miles now, because, like your engine, it has a mild cam and mild valve springs.
