22 replies to this topic

[Bungle]

so why the different grades for different ambient temperatures specified by car manufactures ?

[R1minimagic]

The first number refers to the cold cranking viscosity, colder climates use lower number to ensure the oil is thin and can pump easily from cold.

The higher number refers to the viscosity at 100C (i.e. engine fully warmed up) and is specified to ensure correct protection on the engine.

I think there is some confusion here, the viscosity grade has nothing to do with how long the engine takes to warm up!

[Frisco]

Because if its colder outside you want the oil to be thinner to start with so your not starving the engine of oil while trying to pump tar around the engine.
The heat will still rise the same but obviously the colder it is to start with the longer it takes to get up to proper temp

[R1minimagic]

Agreed!

Also, if you have too thick an oil for the climate, the engine will not even start in the first place as it will turn over too slowly. Any engine has to be doing something like 70-80 revs per minute on cranking to start.

[Bungle]

so it's more to do with cold runnyness that warm running

thanks

[R1minimagic]

Things are also changing w.r.t the way OEM's specify which oil viscosity grade to use.

These days fuel economy and CO2 emissions are very important, so you will notice they are specifying lower viscosity grades as standard to gain those few extra mpg!!

[jaydee]

So basically 10w40 and 20w50 have the same heat capacity?

[R1minimagic]

It would be virtually the same yes