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If oil is inside the air box, it can saturate the air box and drip out. If any oil at all is inside the air box you need to have the engine checked for blowby. If the engine oil is on the outside of the air box only then odds are you have a seal or a gasket failing, and it needs to be cleaned up and the leak needs to be isolated, so it can be determined where the oil is coming from, so repairs can be made.
Blowby is caused by engine wear and is a bad sign. So if you have oil inside the air filter housing it is likely blowby, and you need to have a leak down test to confirm whether blowby is the problem. A quick test for blowby is to remove the oil cap (more evident when the engine is hot), with the engine running. If there is an outward flow of smoke or air you have blowby. Sometimes oil will splash out when checking for blowby with the oil cap off with the engine running, so be ready for that.. What you should see on a healthy engine is a constant rapid inward, and outward pulse. You can also place your hand over the opening and see if it builds pressure, which would indicate blowby. What you should feel with your hand over the opening is a constant rapid inward, and outward pulse, that builds no pressure.
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