What is the difference between multigrade and monograde oils?
Multigrade oils must meet both a W low temperature (Cold Cranking) viscosity requirement and a 100C operating temperature requirement. Monograde oils do not have a low temperature requirement. Therefore, as the temperature decreases, they thicken considerably faster than equivalent multigrade oils, hence they are less efficient (e.g. SAE 30 and SAE 10W-30). This is what the viscosities on the label denote (e.g. 10W-50).
