Unless your vents have some kind of fault, I don’t think you should worry. They aren’t vented to the frame. No design flaw—just an old fashioned, old school air-cooled engine
The engine is just not getting hot enough to burn off the “mayo,” which is an emulsion of engine oil and water vapor.
Warm it up gently for 8 to 10 miles, and then run it really hard at freeway speeds for 20 or 30 miles. That process will burn this mayo out.