An interesting thought, rusty.ute. The breather hose enters the air cleaner outlet duct (carburetor inlet duct) quite a long way below the air cleaner body. Hence I don't see how oil mist could get up to the filter against the air flow unless there were substantial backward puffs from the intake port, through the carburetor, to carry the mist upward. There is always a small amount of puff-back due to the inlet valve not closing until after the compression stroke has begun. However if it were anything substantial the engine should need to have its oil topped up regularly, and probably there would be a smell of burned oil in the exhaust, and maybe a slight haze of blue smoke.
Having said all that, I don't like the plastic disk Sanli uses in the breather valve. We've already had a thread where it malfunctioned and caused oil to leak out of the breather hose where it connected to the air cleaner outlet duct:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=52741#Post52741
