I run 5-40 Rotella T5 in everything including my zero turn with a 25 horse Kowalski. Never an issue. I do pull the shroud yearly and blow out the crap. Air cooled engines need cooling air....
700 hours on the Kowalski, no issues other than a valve adjustment. Incidently, the Generac extra capacity spin on oil filters fit the Kowalsaki, I use them, change the oil every fall.