Never said it was the leading cause of fuel in the oil just it can be a contributor. Just like if I put it to you this way. If there is NO oil barrier how much easier would it be for the fuel to get past the pistons and rings and get into the crankcase.

Looser tension designed oil and piston rings probably contribute more then the oil itself does until the dilution becomes greater. At that point there will be a compromised oil barrier even if we are only talking about a few microns thick.