What oil viscosity numbers mean
Numbers you find on cans of oil such as SAE 10W-50, SAE 30 are viscosity (or weight) numbers, which indicate the thickness of the oil. The higher the number, the thicker the oil.
SAE stands for Society of Automotive Engineers, which established the grading system and W means the oil is intended for use in cold weather. The W designation is particularly important to keep in mind when temperatures drop to 32" F or below .Above that level, oil having a viscosity number without the W works satisfactorily.
When you start an engine in cold weather, oil with a low viscosity number (for example, SAE 10W, SAE 5W-30, SAE 20W-40) flows readily, reducing friction that might otherwise make starting difficult. Oils with high viscosity numbers (SAE 30 or 40) become thick in cold weather and usually make it harder to start your engine.
However, there is a drawback to using oil with a low viscosity number. As the engine gets hot, oil gets thinner. An oil with low viscosity to begin with may not give the engine the protection it needs.
Since more than one grade of oil is usable over any one temperature range, you have to decide which is best.
One of the most Important points to consider in selecting the right viscosity of oil for your car is the type of driving you do. For instance, 10W oil may be ideal if you live in a cold part of the country and drive under ideal conditions. But if you make long trips and carry heavy loads in the car, you will need the cold-starting advantages of a 10W oil plus a higher viscosity oil to protect engine parts. You would, therefore, select a 10W-30. 10W-40 or 10W-50 oil.
Another factor to think about in choosing oil is whether to buy a multigrade or single-grade product. Single-grade oil is less expensive, but there are other considerations.
Multigrade vs. single-grade oil
Multigrade oils are those that have two numbers such as SAE 5W-30, SAE OW-30, SAE 20W-50. Multigrade oil allows engine starting in cold weather while providing the protection of high viscosity oil when the engine is hot. Its biggest advantage is that you don't have to change oil as the seasons change.
Multigrade Oils contain special additives called polymers. As engine heat rises, the polymers change the character of the oil to prevent oil from thinning too much. Such oils are convenient,too, but no multigrade has quite as much high temperature viscosity as the corresponding single grade. For instance, 10W-40 oil isn't as thick in a hot engine as a single-grade 40.
"You get a little better protection at high temperatures with a single-grade oil." However, to take advantage of this protection, the oilgenerally has to be changed from one grade to another as the seasons Change. An exception to this Involves use of SAE 20W-20 oil.
Incidentally, if your car is burning oil, you can cut consumption until you are able to repair the engine by using a high viscosity single-grade oil.