15 miles how could it be that gunky? Drain the oil put the drain plug back in, pull the fill plug and valve covers and fill it with kerosene (it won't hurt the seals) and let it sit for awhile, drain it and do it again. Drain it.Let it dry, fill with oil fire it off, let it run a bit, drain the oil again. I can't begin to imagine the cost of parts for those.
