The oil tank cap doesn't vent, by design. And lest you be tempted to go down the path of re-designing the breather system as some have done at great expense you should understand the factory system is fine, as long as it's working correctly.
If you haven't yet relocated the vent line from the tank to the cam case (the smaller line, RF of engine), it's worth doing as many cars have that line routed with a low dip just to the rear of the engine that can allow congealed oil foam to block that line. Frequent short drives that do not heat the oil enough to evaporate the inevitable water in the oil can create that oil/moisture foam (looks like frothed mayonnaise) and if not allowed to fully drain back into the cam case when things are hot, can lead to a blockage in the 'dip' when things cool down.
You also want to ensure the short hose from the top of the cam case to the inlet manifold is not blocked, and that the small orifice piece is in place. Also, there is a thin stainless reed valve in the cam case that needs to be functioning, and though rare, I have read of a couple of breakages.
But have you removed the air cleaner assembly to inspect the intake tracts yet? That will tell you if the engine is ingesting oil there. If not, then it's coming in through the guides or past the rings.
