Have the oil sampled for gas.

The common ways for gas to get into the oil is either a leaking mechanical fuel pump or fuel thru the carbs.
If it is the fuel pump, i would temporarily set up an electric pump to se if it leaks in that way. It would be a torn difam or seal in the pump. And I would do it with the carb fuel line removed and plugged off or routed to a catch can.
I had it happen to a 350 chevy once with the mechanical fuel pump. That was a neat and exciting explosion. It pushed the dipstick completely out of the tube, and made a pair of rare valve covers, one of a kind!!