2 possibilities - if there is a fuel pump fitted, chances are the diaphragm is ruptured, allowing fuel to be drawn into the crankcase through the vacuum pipe.
If not, it will be happening when the engine is at rest. The inlet needle is leaking, allowing fuel to enter the engine, overflow the float chamber, and enter the cylinder through the open inlet valve, where it ends up in the oil.
phil_saunders(a)bigfoot.com
Phil Saunders, June 2012
