hey dave check your fuel pump!! if you carb is that rich to thin the oil in just a short drive 8 - 10 miles you would have to be just about pumping gas out the tailpipe. you must be pumping raw gas down into the pan somehow. a fuel pump is a cheap easy fix and also the only way raw fuel can get down into the engine unless its literally running out of the carb
if the engine runs good and does not show signs of running rich i dont think the carbs the problem.