The first thing you will notice when you look up different brands of engine oil will be the number that is prominently positioned on the bottle, for eg. 10W-30 or a 5W-40 etc.

These sets of numbers are viscosity ratings developed by the Society of Automotive Engineers (SAE) in a control laboratory environment for both engine and transmission oil. Oil viscosity refers to how easily oil pours at certain temperature.  The lower is the viscosity rating, the ‘thinner’ the oil. The higher the viscosity rating is, the ‘thicker’ the oil.

There are 2 types of SAE classifications: monograde and multigrade. Almost all engine oil today are multigrade as they offer better properties across a broader temperature range. They are widely used as they allow easy starting and pumping at low temperature and yet sufficiently thicken at high temperature to lubricate effectively.

What does 10W-30 means then?

The “W” stands for winter. It indicates the oil meets certain viscosity requirements for winter and cold temperature operation.

The number in front of “W” is the low temperature viscosity. It indicates the performance of the oil when the engine is starting up. A “thinner” oil when cold protects engine components better as it moves around the engine quicker.  This is especially important in climate countries as excessive “thickness” during winter causes the oil to thicken and starting the engine will be harder as more energy is required to draw the oil from the oil pan.

The second number is the viscosity rating at high temperature. It is the “thickness” of the oil under operating temperature. The higher the number the more resistant it is to thinning. Thicker oil generally seals better and maintain a better film of lubrication between moving engine parts.