So I look at a data sheet for a 5W-30 and it tells me that the cst@40C is 65, and the cst@100C is 11. So far, so good.

I then look at the viscosity chart on this website and the numbers I see don't make sense for the 5W/40C end of things. The chart says that a 5W oil at 40C has a cst of around 18 to 20 - not the cst of 65 shown on the data sheet. If I take the cst of 65 @ 40C and move up the chart, it says it's equivalent to a 20W oil @40C.

So is the 5W-30 in my example really a 5W-30, or is it actually a 20W-30 ? The numbers are pretty similar for all the 5W-30s. In fact, this descrepency exists for all multi-grade oils I looked at. The numbers all match quite well on the 100C end of the scale for the listed viscosities. It just the cold end that doesn't make sense to me.

What gives ?

Phil