A bad fit or the wrong rubber seal at the filler cap would allow a significant leak; I would investagate that first. I have had to slightly bend the tabs on older bricks to get the cap to fit tight.

Your car does not have a flame trap but all of the plumbing normally associated with the flame trap is there and needs to be free flowing.

Check the small hose that runs from the intake to the flametrap like holder. Is the hose clear and is the nipple into the intake open?

The oil seperator can be checked by attaching a longer hose to it and blowing on it. There should be no restriction. If it needs to be cleaned or replaced it is best to remove the intake. My suggestion is to purchase a name brand intake gasket. I used one of the ones available at the popular online parts supply and it started breaking up in less than 5000 miles (actually sucked a piece in..).

You did not mention how many miles are on the engine but high mileage cars might have enough blowby the rings to pressurize the crankcase.

You should be able to place your fingers over the oil fill opening and feel a slight vacuum at idle.

Good luck

Randy