Monograde oil carries a single SAE viscosity grade such as SAE 30, 40 or 50, with no viscosity improver additive to widen its temperature range.

It is still specified for plenty of applications: older engines built before multigrades became standard, agricultural and construction equipment, stationary engines and generators, some older transmissions and hydraulic systems, and small air-cooled engines. Without a polymer viscosity improver to shear down in service, monograde oil holds its viscosity predictably under sustained load, which is why it persists in steady industrial duty.

The trade-off is cold starting. A monograde heavy enough to protect at operating temperature circulates slowly on a cold morning, so it suits warm conditions and constant-speed machines rather than stop-start road use. Check the equipment manual for the grade and service level required.