How to decode the viscosity index

After the SAE abbreviation we see several numbers separated by the letter W and a dash, for example 5W-30 (for the all-season oil that most car owners use). Without going too deep into the physics and terminology (that's further below), the marking can be decoded like this:

5W – this is the low-temperature viscosity, meaning a cold start of the engine is possible at a temperature no lower than -35°C (that is, you need to subtract 40 from the number before the W). This is the minimum temperature for this engine oil at which the engine's oil pump can still pump the oil through the system without letting the internal parts run dry. It has no effect whatsoever on how a warmed-up engine performs.

If you subtract 35 from that same number instead (in this case that gives -30°C), you get the minimum "cranking" temperature of the engine. Obviously, as the temperature drops the oil gets thicker and it becomes harder and harder for the starter to turn the engine over on a cold start. But this is an averaged figure, and the real picture depends heavily on the engine itself, which is why it's so important not to stray from your car manufacturer's viscosity recommendations.

That's it – beyond that, the first number before the W means nothing at all, and it has absolutely no bearing on how a warmed-up engine runs. So if you live in a region where the winter air temperature rarely drops below -20°C, virtually any oil sold on the market will suit you on this parameter. The real question is the condition of your starter and battery – if they're already a bit worn out, they'll unquestionably find it easier to start the engine at -20°C on a 0W-30 oil than on a 15W-40.

The second number in the marking is far more interesting – the high-temperature viscosity (in this case, 30). Unlike the first number, it can't be translated into plain language so easily, because it's a composite figure indicating the minimum and maximum viscosity of the oil at operating temperatures of 100-150°C. The higher this number, the higher the engine oil's viscosity at high temperatures. Whether that's good or bad for your particular engine is something only the car manufacturer knows.