Engine Oil Thickening in High-Speed Passenger Car Service

Excessive engine oil thickening has occurred in some cars during the past two years. Apparently, this thickening was caused by prolonged exposure of the oil to high temperatures. Temperatures have been generally increasing in recent years because motorists are generally driving faster, and higher loads imposed by trailers and air conditioners are more common. The high-speed track test reported in this paper showed that oil oxidation and nitration cause thickening, but antioxidants control it; thickening is not necessarily accompanied by heavy sludge and varnish deposits, and it occurs more readily in some cars. To diminish the occurrence of thickening, higher quality oils are needed.