[Kenth]

To attain the relevent sae number the oil has to be at 100°C.

SAE 30 11cst approx
SAE 40 14cst approx
SAE 50 18cst approx

Centistokes (cst) is the measure of a fluid's resistance to flow (viscosity). It is calculated in terms of the time required for a standard quantity of fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid.

As viscosity varies with temperature, the value is meaningless unless accompanied by the temperature at which it is measured. In the case of oils, viscosity is generally reported in centistokes (cst) and usually measured at 40degC and 100°C.

SAE 60 is in fact 24cst viscosity at 100°C!

This is 33% thicker than an sae 50, 70% thicker than an sae 40 and over 100% thicker than an sae 30!

So, what's the problem with this thickness?

Well, this is measured at 100°C and at lower temps (70-90degC) all oils are thicker than at 100°C so the problem is compounded to some extent.

The downsides of such a thick oil (when not specified) are as follows:

Additional friction, heat and wear.
A reduction of BHP at the wheels
Higher fuel consumption.

The 10w part is the cold start viscosity and it is the same regardless of the specified operating temperature viscosity, in case anyone thought otherwise.

FWIW