Jerry,
10w-40 right out of the can looks pretty thin. If you have enough fuel in your motor oil to affect oil pressure, you should be able to smell it quite easily.
There's a procedure where you heat a small spoonful of motor oil with a candle while holding another flame over the sample. Any fuel in the oil will produce vapors that will flash and burn long before the motor oil itself reaches it vapor point.
It seems like it would be easier to pull the fuel pump and check it. The pumps held together with screws can be busted apart and the diaphragm visually inspected for tears.
Try the smell test for a gas odor first.
Stuart
