Multigrade Oils
Oils can also be multiviscosity or multigrade. They contain additives that permit the oil to flow steadily across a large temperature range. In other words, the oil does not excessively thicken in cold weather nor thin out in hot weather. Multigrade categories include SAE 5W-20, SAE 5W-30, SAE 5W-40, SAE 10W-30, SAE 10W-40, SAE 10W-50, SAE 20W-40, and SAE 20W-50.
For example, a SAE 10W-40 oil will act like a SAE 10W oil when cold, but a SAE 40 oil when hot. Recalling that the lower the viscosity, the thinner the fluid, this oil will remain thin in cold weather for easy starting and circulation after the engine is started. As the engine gets hot and reaches operating temperature, the oil will thicken slightly to maintain sufficient lubrication film strength. Multigrade oils are ideal because they can be used in all seasons.
Consult your owner's manual to determine the correct oil viscosity for your engine. Many manuals recommend SAE 10W-30 if outside temperatures are generally above 0° F. For outside temperatures generally below 0° F, SAE 5W-30 oil should be used, even if just for the winter season. Viscosity requirements are usually determined by the coldest outside temperature in which the car will be driven. You should use the lightest grade oil sufficient for the coldest temperature conditions to be endured. The following table can be used to select the appropriate viscosity (grade) oil:
APPROPRIATE OIL VISCOSITY
Note that SAE 5W oil offers great cold weather protection, but it is too thin to be used for long distance highway driving. The oil would be squeezed from the parts. SAE 50 oil should be used instead for long distance, highway driving. The same oil should be employed in older, worn engines to reduce oil consumption.
