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  • August 2019
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Car Engine Oil Guide

All The Oil Information You Need In One Place

Selecting the correct oil for your vehicle can be a bit of a mine field. Some basic understanding of what the specifications are, and what the numbers printed on the can mean will be a help to selecting the correct oil.

Just about all modern vehicles use a multigrade oil, so what does it all mean?

If you see an expression such as 10W-40, the oil is a multigrade. This simply means that the oil falls into two viscosity grades, in this case 10W & 40. This is made possible by the inclusion of a polymer, a component which slows down the rate of thinning as the oil warms up and slows down the rate of thickening as the oil cools down. It was first developed some 50 years ago to avoid the routine of using a thinner oil in winter and a thicker oil in summer.

For a 10W-40 to attain the specification target a 10W (W = winter) the oil must have a certain maximum viscosity at low temperature. The actual viscosity and the temperature vary with the viscosity grade but in all cases the lower the number, the thinner the oil, e.g. a 5W oil is thinner than a 10W oil at temperatures encountered in UK winter conditions. This is important because a thinner oil will circulate faster on cold start, affording better engine protection.

For a 10W-40 to attain the other specification target SAE 40 the oil must fall within certain limits at 100 degC. In this case the temperature target does not vary with the viscosity grade, if there is no “W”, the measuring temperature is always 100degC. Again the lower the number the thinner the oil, a 30 oil is thinner than a 40 oil at 100 degC, which is typical of maximum bulk oil temperatures in an operating engine.