Check any crankcase venting used on your engine (pcv valve, breather system etc) if any are plugged it will develop positive pressure inside that will drive oil out. If venting is clear, you can have a worn or broken piston ring which will allow compression to enter oil pan which cannot be eliminated by vent system. Generally though you would have plug fouling on any cylinder with that problem. Overall ring wear from time and miles can also contribute to this condition...slight wear in all cylinders can amount to a certain amount of "blowby" which in total cannot be eliminated by venting system. In that case, if condition is not severe, sometimes changing to a high mileage type oil, a synthetic, or using an additive like marvel oil can slow down, stop or slowly reverse some of the problem.
What not to do: Do not use a single grade or heavier single grade oil...in general this will worsen the problem.
Pcv valve
Is the crankcase overfilled? is the crankcase vent clear and the PCV ok and working? do u have excessive crankcase pressure caused by leaking piston rings, this is common on high mileage engines, esp diesels.
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