There are two anti-drain back valves in the Yb engines lubrication system, one is in the oil filter the other is in the cylinder heads main oil supply gallery. Typically if these are faulty and allowing the oil to drain back you will notice a delay in the oil pressure rising on startup and tappet noise/ big end rumble until the oil has refilled the filter and galleries etc and normal pressure has been reached.
You need to set the oil level after the engine's lubrication system has been primed.
If you suspect there may be a drain back issue with aftermarket breathers and modifications and you have no idea how much oil is actually in the engine it may be wise to drain the oil and refill it with the correct quantity and then re asses