Could you enlighten me. As I am of no car enthusiast, why suddenly my old car has this problem? All these 20 over years, I do not have lubricant in the air filter compartment. I suspected the one-year old overhaul job is the crux of this oily issue. I am also told by other car mechanics, there could be problem with the piston ring.
When most people do a "full overhaul", the head is usually skimmed a bit so your compression is higher. I don't know if they changed your piston rings so assuming it's a proper full overhaul, those piston rings would be changed so blow-by gases wouldn't be the cause of your oily situation.
For emissions purposes all cars usually recirculate the blow-by back into the intake, so if that blow-by is oily, it'll just gather in your intake and result in lots of oil vapour and all caught in the filter or intake area. A higher compression also partially contributes to more pressure in the system aggravating the whole vapour situation. That's why adding a oil catch tank (or in your case a cheap petrol filter acting like an oil catch) stops all that oil vapour from recirculating back in to your intake.
Regarding the piston rings - full overhaul should have replaced those rings. If he's saying there's a problem then you better question the mech why wasn't the piston rings changed in the full overhaul. If it's just a top overhaul then those rings wouldn't be touched. A worn piston ring will usually have bluish or blackish exhaust smoke because of the engine oil burning off due to the bad seal from the piston ring.