What is the effect of excessively high oil viscosity on cold-start engine wear?

Excessively high oil viscosity at cold-start temperatures significantly increases engine wear due to reduced oil flow and delayed lubrication. Viscosity is a measure of a fluid's resistance to flow. At low temperatures, oil becomes thicker and more viscous. Using an oil with a viscosity grade too high for the ambient temperature impairs its ability to flow quickly to critical engine components during startup. This delayed oil flow results in a period of oil starvation, where parts like bearings, pistons, and valve train components are not adequately lubricated. During this period of oil starvation, metal-to-metal contact occurs, leading to increased friction and wear. The high viscosity also increases the load on the oil pump. The pump has to work harder to circulate the thicker oil, which can cause excessive pump wear and even pump failure in extreme cases. Furthermore, high viscosity oil can have difficulty passing through the narrow oil passages within the engine, exacerbating the oil starvation issue. The increased friction from the high viscosity oil also generates more heat, which, while eventually reducing the oil's viscosity, initially contributes to higher engine wear. For example, using a 20W-50 oil in an engine designed for 5W-30 in sub-zero temperatures will result in significantly increased wear during cold starts because the oil will be too thick to properly lubricate critical parts quickly enough.