Every so often, I will hear about a problem with a person having oil in their lawn mower engine carburetor. This is a sign that something is amiss; however, it may or may not be serious. The most common reason oil finds its way into the carburetor is if the lawnmower has been tipped over, whether by accident, through storage, or to sharpen the blades. If this has happened, try taking the carburetor off the engine and cleaning it extensively. Replace the carburetor, but do not start the engine. First replace the air filter, and then it’s also a good idea to replace the sparkplug. Drain the oil out of the engine, and then replace the oil with your standard 2 cycle oil or whatever your lawn mower uses.
This is the most common issue and the most common fix, however it could be an incorrect diagnosis, and should this situation happen again, consult a mechanic.