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.
Ive seen this exact same thing happen to engines even with proper oil changes . The company I used to work at back in the 80s used nothing but Valvoline, And after pulling the valve covers on one truck and I couldn't see the rockers arms,
Ive never used Valvoline since
