It is burning oil. Either because you have overfilled the engine, or at sometime recently you have tipped it on its side causing oil to migrate from the sump and into the exhaust. Check the oil level and if too high, then obviously remove it until the correct level is achieved. If you have tipped it on its side, then keep it running for a few minutes until the excess oil in the exhause has burnt off.
pleasant, September 2008
