Interesting Oil Facts #2

At which temperature does an oil flow better?

There is no temperature where oil suddenly starts to flow better.

A 10W/40 for example will flow between -25C and 100C or more, but there is some difference in the rate of flow! (True viscosity at -25C is about 7000 Centistoke(cSt) units, dropping to 14 at 100C. ‘Viscosity’ is really just another name for ‘rate of flow’.)

The correct question is at what (sump) temperature is an oil at a viscosity that suits a modern high-RPM engine? Present day designs are happy on an oil viscosity of 10 to 15 cSt. (But many are OK on less than 10.) 30cSt is too high at high RPM. It can lead to foaming, air entrainment and cavitation.

Temp. for 30cSt (DegC)..........Temp. for 15cSt ............Temp. for 10cSt

5W/40.........71.............................90.................................117

10W/40.......70.............................99.................................118

10W/50.......80............................109................................130

10W/60.......89............................119................................142

As you will see from the above table a 5W/40 or a 10W/40 is perfectly adequate for all engines except those that run unusually high temperatures (120degC plus)

Cheers

The Opie Oils Team