I also take viscosity into account when considering which is more likely to cling to metal surfaces after the engine is shut down. If you were to heat 2 different weights of oil (let's say 5W20 and 10W30 to 180 degrees) and dip a piece of billet aluminum in each. Which oil is likely to leave an thicker film on the aluminum when you pull it out of the oil? Which one is likely to drain off and leave little or no film behind? Will they both drain off to the same film thickness? If so, which one will take longer to do so? Discuss.