sounds like bad rings. My other thought is similar to what happened to the 350 in my old chev truck. We were having ignition issues and were cranking it over and over and over again while trying to fix it. Eventually, all the fuel that was being pumped into the cylinders built up and got around the rings and down into the oil. Granted, that was on a 50 year old, non efi truck, but the symptoms are similar. I'd suggest having a compression check done on the motor. If the gas is diluting the oil, you'll probably have some sort of compression loss in the cylinders. Also, have you checked your spark plugs and plug wires lately? If you're getting weak spark, it might leave some unburnt fuel in the cylinders.
I'd be careful with the truck, though. A friend of mine told me about a car he had where fuel was getting down into the oil. Somehow, the fuel that was down in the crankcase ignited while he was driving it and blew out the bottom of the oil pan. I don't know if this kind of thing happens a lot, but it seems dangerous.