The "plastic valve" you refer to is the idle speed air distributor, which is part of the PCV system. It has no moving parts, but has a 1.5 mm metering orifice. You should probably remove and clean it.

When there is manifold vacuum blowby gases are metered to the idle air system via the metering orifice. At WOT blowby passes directly to the air cleaner interior and then into the engine

Oil in the air cleaner may mean the liquid vapor separator in the cam cover is broken. You will need to remove the cover to inspect it. Some are replaceable, some are not. There have been recent threads on this issue.

Also renew any deteriorated PCV system hoses.

Duke