Well
I'm not convinced it has excessive blow back,---------- yet.
pulling an oil fill cap off on a warm/hot engine will usually give you oil in yer face!
kind of like looking @ a nozzle of a garden hose and pulling the handle, then thinking Duh,that was stupid.
I hear the rattle John hears, but be advised the valve train on those things are noisy,some more than others.
I'd go slow in calling it a bad engine, till you warm it up and check things out.
With the air cleaner element removed, a hot engine will puff a small amount of air but not slobber oil out of the crankcase vent as John said .
Go slow and methodically.