One of my first vehicles was a '64 Pontiac Catalina. The engine oil was thick & gunky. I flushed the engine with automatic transmission fluid, then took the top of the engine down to the cylinder head gasket; soaking all the components (pushrods, rocker arms, etc.) with solvent & cleaning them nice & shiny. Reassembled according to my trusty Hanes or Chilton; torqued to spec, yada yada. Drove it to the gas station to proudly fill up with gas & go for a fun day of cruising the country back roads. As I arrived at the pump, the oil dumped & an impressive cloud of smoke issued forth, flowing along the ceiling of the pump canopy, curling up & along its edges; taking my day's aspirations with it. Unbeknownst to me, the previous owner had used 5W20 because the oil pan gasket & other seals were shot. Learned several lessons that day. =D