ANSWER: Fuel dilution can lead to much increased wear, component seizure, increased oxidation and the potential for fire. Ten percent fuel contamination can remove 27 percent of piston ring metal in 100 hours of operation.
Question & Answer from Noria Machinery Lubrication, Lube Trivia <http://www.machinerylubrication.com/t/MLT1-2013-11-27>
While Fuel Dilution is not usually a problem with properly operating gasoline and diesel engines, there is an exception. Small diesel engines starting in 2007 that use a particulate filter that is “Regenerated” by injecting diesel fuel into cylinders on the exhaust stroke tend to have a higher level of fuel dilution than what would have been considered normal for previous engines. This problem is created when some of the unburned fuel washes past the piston rings and gets into the oil in the crankcase. For these engines AMSOIL does not recommend extending the oil drain interval unless you do oil analysis at the manufacturers recommended change interval. Oil analysis will give you a snapshot of the oil at the time of the sample and let you know if the oil is fine for continued use or if it should be changed and why.