Oil in the air filter is usually cause by the engine being over filled with oil or some one tipped the mower onto its side. You will need to check the oil level and remove some of the oil if it is over filled. After adjusting the oil level, the air filter has to be replaced. I recommend starting the engine before installing the new air filter to make sure oil is not going to soak the new filter. If oil still comes out of the small tube by the air filter, it is a sign if another problem, the crankcase breather is faulty or the engine is worn out.
Here is a link to order any parts you need.
Sears Parts Direct
If you do not feel confident repairing this problem yourself, then you can have it repaired by a Sears technician. Here is a link that will provide the nearest drop-off locations:
Sears Home Services