Oil vent trap

It sounds like your Oil is Foaming!
What Oil are you using?
This can happen if there is Fuel in the oil.
The fuel boils off when the Oil gets hot causing the Oil to Foam.

Is the screen installed in the oil tank? It removes air bubbles from oil returning from engine.

I use the expensive recomended oil in both my engines. I have owned the fox for almost 4 years and it has always done it and since both planes do it I assume it is normal . I also see a liitle oil leak around a cylinder and woner what cuaes that?

I never see any sign of fuel in the oil changes? They both have new top ends.

Yes

Time to get Serious!
Can you reproduce the problem on the ground?
Top off the oil and start the engine while monitoring the Oil Vent line.
Does it Vent Oil immediately?
Does it vent Oil As it gets Hot?
Does it Vent at Idle?
Does it vent at WOT?
Does it Vent Oil at all?

If you can not get it to vent on the ground, it might be an aerodynamic issue.
Mount a GoPro camera to monitor the hose in flight and see when the vent happens.
Is your vent line flush cut or cut at an angle?
Which way does the Open-End point relative to the airstream?
Try pointing it in a different direction. Make it longer/Shorter.
It may be in an area of a vortex and experiencing airflow opposite to what you would expect.