Usually most of the oil leaks down past the rings by spring. If it started, I doubt that it is still the oil in the cylinders. Once the engine is running it will spit anything in the cylinders out pretty quickly (a couple minutes at most). I'd look towards the carbs / intakes.

How long does it run before the plugs foul ? Are all four plugs equally fouled ? Is the exhaust very smokey ? Is there raw fuel In the cylinders ? Did a mouse build a nest in the air box ? Do your carbs have overflow tubes or float bowl vents ? If so, is there fuel there ?

Greg