Do thicker oils provide better protection than thin oils?
No – typically you only see the oil when filling and draining the engine. These operations are usually carried out at room temperature or with the engine/oil warmed, but not hot. Under these conditions, the multigrade oils will appear thinner since they are formulated to remain fluid down to low temperatures. However, they are also designed to remain sufficiently thick at higher temepratures to protect the engine. At 100 C SAE 15W-40 and SAE 40 oils will have similar viscosity – BUT above 100C the multigrade oil will remain thicker than the SAE 40 providing even better protection than the monograde oil.
