There is a product here for spot cleaning called K2R, it's sold in some of our super markets in the cleaner section, it works pretty well. Acetone and a rag helps. These things help but once the oil gets into raw wood that hasn't been fuel proofed nothing really gets it all out. Some people use crushed kitty litter too or baby powder. A lot of ways but none of them fool proof. It takes a very long time for the oil do start doing real damage and the owner will probably have several new planes but the time he will see any real problems. Always fuel proof inside the plane as well and as far back as you can just to be on the safe side.