It depends on how hot the engine runs. Running up at 200 the oil gets black in a few days. I would say 2,000 miles at the most. Running below 180 my oil never gets black but gets to a slightly dark yellow. I still change it in 3,000 miles. I use castrol 20/50 in my 39 for over 30 years since start up, it still has 40 Lbs oil pressure is quiet and and don't burn a drop of oil. G.M.