It is normal for a small amount of oil (usually a smokey vapor) to pass from the crankcase breather into the air filter. However, if your having an excessive amount of oil then the the stem seals on the valves have hardened and it is allowing oil to pass along the valve stem through the valve guide through the breather and into the air filter. The remedy is to replace the stem seals under the valve springs inside the cylinder head.
One other thing that will do it is if someone overfilled the engine crankcase with oil. This will cause massive amounts of oil to be pumped into the air filter.
Thanks for using FixYa.
Kelly
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