I think about 2000cc? It should be stamped on the crankcase cover. I used to use synthetic 20w-50 (i live in CA at sea level) but had probs w/slipping gears and switched to Rotella T, 15W-30 or 40 i think. It´s formula is closer to the blends available back in the day. I´ve a ton of ´79 xs400 parts, carbs parts and lots of trims (lights, electricals, etc.) give me a holler if you need anything. -Patrick Kennedy