suckin oil into breather and air fliter

:swear: I have a 2.5 litre 4cyl. 1992 sonoma,and I have to change the air and breather filter once a month due to oil being sucked into it.I have changed the valve and also cleaned the egr valve to help with the power.If it's the rings,pistons or valves will any oil additive for worn engines help at all.Also the gas mileage is horrible,it's gone from 30 mpg.to maybe 18.Also,could you have a blown head gasket without losing any water?

I would make sure the PCV valve has plenty of good engine vacuum it.
Check the hose to from engine and where it goes into engine or intake.
And use a good AC Delco pcv valve.
Is the engine building a lot of blow by pressure in it?
Has the oil got gas in it or broken down from using more fuel or bad gas mileage?
Go over engine running tune up end.
Plugs and engine pcm sensors.
Any codes or check engine lights?

Most oil additives are snake oil.
A good engine and good oil does not need additives.
If the engine is about worn out sometimes a little oil honey like STP will build up or stiffen up the oil base and help a little.

On the head gasket there is several ways a head gasket can leak.
And they could leak without losing water.
But there would be other symptoms on head gasket leakage like.
Engine heating up or smoking out exhaust or engine miss or coolant in oil or external oil or coolant leakage.