Saw that a lot in the 70s with regular oil changes of quaker state and z7 pennsoil. Oldsmobiles were the worst, cylinder head drains would plug. We used to pour 3 gallons of kerosene in them, run them, and change the oil to valvoline or texaco, never a problem after that.
Kerosene
That sure would LOOSEN IT UP but then it drops into pan THEN blocks the pickup
Never a problem, scrape out what you could, jack the front up on the rack to about a 30 degree angle, let it drain overnight, chunks came out. Never lost a motor, one olds went another 100,000 after that. 318s had an issue with the plate on the bottom of the intake, oil would coke on the intake after the heat riser stuck shut. I saw one so bad that you could not see the cam and the plate and 2 rivets were laying in the motor. Same deal, clean, kerosene and a few oil changes and it never quit till it went to the junkyard.
