It may be a little known fact, that during WWll (and other times) aircraft piston engines used in very cold climates called for dilution of the
oil with fuel to aid cold starting. I don't know the percentage of fuel used, but I'm sure it was small, and the fuel vaporized out as the engine warmed up. You can Google it for more info. The point is that a small amount of fuel in the
oil is tolerable, but not worth the chance under normal circumstances.