In an engine sans oil filter, I am inclined to FLUSH contaminants out of the crankcase, earlier than what OEM decrees, WHEN...
The oil turns from yellow to amber or darker
Smell the oil. If it starts to smell like gasoline
I installed a 22-liter oil pan, pickup, C-500-L Luber Finer bypass element, and belt driven tiny gear motor pump on my Kubota. It uses Delo-400 15W-40 petroleum lube oil.
IMHO air-cooled engines can run "Hotter-Than-The-Hinges" in certain environments and loading combinations. This is where a good synthetic may be appropriate.