It's a question I get asked all the time so I thought that I'd clear up one of the most common misconceptions about oil viscosity.

"What's the difference between a 10w-40 and a 5w-40?"

I read the following comments all the time!

"You must not use 5w oils"

"5w oils are too thin"

"10w-40 is ok but don't use 5w-40!"

Some facts need to be straightened out here and hopefully I can do this without losing those that are interested.

So what is the difference between a 5w-40 and a 10w-40 @100degC?

NONE!

SAE determines the viscosity (+/-) that the oil needs to be at 100 degC and these need to be met in tests to give the oil it's API rating (xw-xx or xxw-xx).

They are as follows: (within a margin)

SAE 30 = 11.00cst
SAE 40 = 14.00cst
SAE 50 = 18.50cst
SAE 60 = 24.00cst

Compare these SAE 40 oils at 100degC (0w, 5w, 10w, 15w)

Motul 300V 5w-40.................Viscosity = 13.80cst
Motul 300V 10w-40................Viscosity = 14.00cst
Silkolene PRO S 5w-40............Viscosity = 14.89cst
Motul 8100 X-cess 5w-40........Viscosity = 14.00cst
Fuchs Titan Supersyn 5w-40....Viscosity = 13.60cst
Castrol Performance 10w-40.....Viscosity = 14.50cst
Silkolene XTR 10w-40 (semi).....Viscosity = 14.70cst
Mobil 1 0w-40........................Viscosity = 14.30cst
Motul 8100 0w-40...................Viscosity = 13.30cst
Silkolene Turbolene D 15w-40....Viscosity = 14.40cst
Total Quartz 15w-40...............Viscosity = 14.50cst

These figures are not by chance and include synthetics, semi-synthetics and mineral oils they are all in the range to be labelled a Xw or XXw-40 multigrade oil.

So, what's the "W" number all about then?

It stands for "winter" not "weight" as often confused and called on many U.S. articles on oil!

This number is the "cold crank" viscosity and nothing to do with the oil viscosity when the engine is up to temperture. These numbers are related to the oils ability to operate in cold temperatures. 0w oils were originally designed to operate in arctic climates that's why they operate at ferrous monkey endangering temperatures of -35degC and below!

The benefits of the lower viscosity oils (0w and 5w) is that they flow more easily and quickly when cold and therefore protect the engine better on cold start when 80% of the engine wear occurs.

So, yes it's true an sae 40 is an sae 40 when hot whether its a 0w, 5w, 10w, 15w or whatever and that's a fact. The same goes for 20's, 30's, 50's, 60's and so on.

Hope this helps.

Cheers
Simon