Understanding Engine oil grades

Try Googleing for SAE oil grades explained.

In a nutshell, the number is a measurment of the viscosity (which is a measurment of the oils restriction to flow, measured on a viscosity gauge).

In the 5W/40 example, 5 is the viscosity of the oil at 20deg C, and the 40 the viscosity of oil at 60deg C (I think those are the right temps). It doesn't mean the oil gets thicker as it warms up, but it acts like a standard viscosity would at that temp (ie it acts like a 5 weight Viscosity oil at 20deg, and a 40 weight viscosity oil at 60deg).

This is all done by the adding by adding viscosity stabilizers to the oil, so that it stabilizes the oils viscosity. I know this is a bit of a contradiction, but it means the oils viscosity doesn't vary as much over temperature changes, but due to the way the viscosity of engine oil is measured/written it would appear it changes

Try this one

I beg to differ!

In 5W/40, 5 is the oil *grade* at 0deg C and 40 is the oil *grade* at 100deg C. Standard oil grades were devised in 1920s by the SAE. A 'standard' oil with a fixed grade will have have a standard viscosity curve with temperature, rather thicker at 0deg C and thinner at 100deg C. That is, much higher viscosity when cold than when warm. The standard oil grade was of course an idealisation of typical native oil behaviour. A higher grade oil would have the viscosity curve shifted upwards, that is higher viscosity at fixed temperature. Yet the viscosity will still drop steeply with temperature.

Then multi-grade oils were introduced. The purpose of that was to reduce the viscosity variation between cold and warm by adding viscosity stabilisers.