its not supposed to get by the valve, though it seems it always does. its quite a common problem, that the valve fails and fills your primary full of oil. check out the stickies on it, some guys block it off completely, though i cant recommend that. its actually a one way valve to allow air into the crankcase from the primary when the pistons are ascending to help with scavenging the oil in the cases. it is my opinion that if that valve is blocked it increases pumping losses, by forcing the pistons to pull against a vacuum.
Guys have been blocking it for decades with no performance issues whatsoever...
...It's a simple procedure that anyone can do.
You can either weld it shut or block it off by tapping the valve's center orifice for threads and then inserting a brass screw into that orifice.
Once it's in there, covered with permanent-strength Loctite, simply distort the screw's head by peening it.
To replace it you have to pull the primary drive. To get it out you have to unscrew it. It is staked in place to prevent it from loosening. I got mine out by drifting it around in a circle with a punch. It was a pain.