My 250 is a newer engine then yours, but still 45 years old. It has had so many different grades and types of oil I lost count and still, it runs strong with no problems. Right now it has 5w20 in it.
If this is a used, stock engine, run any oil like, 10w30 or straight 30, 40 weight if its real hot where you live. The additives are for rebuilt engine with new cams or engines with very high lift cams and big valve spring pressures.
Joe
