Oil residue on the bottom of the plane is normal and a good thing, it means you have enough oil in your gas to adequately lubricate, cool,and protect your engine. Type and brandof oil, fuel/oil mixing accuracy, proper mixture adjustments andoperatingtemperature are all key to minimizing the oil residue but not eliminate it completely. New engines will also leave more residue behind as they digest a little bit more fuel than a fully broken in engineand leave some ofthe excess on the bottom of the plane. It will get less and less as the engine breaks in and mixture adjustments are leaned out. If there was no oil on the bottom of the plane, that would be of much more concern.