If an engine is operated at higher than the specified temperature limits, lubrication troubles will develop. High cylinder surface temperatures will reduce the viscosity of the oil. As the cylinder lubricant thins, it will run off the surfaces. The resulting lack of lubrication leads to excessive piston and liner wear. On the other hand, if the engine is operated at temperatures that are below those specified, viscosity will be increased, and the oil will not readily reach the parts requiring lubrication.
Oil plays an important part in the cooling of the piston crown.
If the oil flow to the underside of the crown is restricted, deposits caused by oxidation of the oil will accumulate and lower the rate of heat transfer. For this reason, the under-side of each piston crown should be thoroughly cleaned whenever pistons are removed. While insufficient lubrication and uneven cooling may cause ring land failure, excessive oil temperatures may cause piston seizure. An increase in the rate of oxidation of the oil may result in clogged oil passages or damage to piston pin bushings. Seizure and excessive wear of pistons may be caused by improper fit.