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This is a hot topic, i wrote out this explanation on another forum, as i tried to understand it in the simplest terms, i hope this may be of use here to.

SAE is the oil ratings standard at which our oils are measured against, for EG 20W 50 which is the measurement of COLD pumping and
cranking at a certain temperature point, then the second number is Viscosity rating @ 100 Deg C.

Cranking and Pumping are measured in centipoise, the oil has to meet certain measurements to achieve a rating number.

The Second number is measured in Kinematic centistokes, this is measured @ 100 Deg C and has to meet a MIN and MAX range to determine
it's rating number.

Centipoise as i understand it is related to centistokes, 1 poise of fluid has the density of 1 gram per cubic centimeter and the
kinematic viscosity of 1 stoke, 1 stoke is 1 centimeter squared per second.

So For eg the stock oil 20W 50

20W(Winter or Cold) Rating is 4500 centipoise @ -10 Deg C While Cranking and 60,000 centipoise @ - 20 Deg C while Pumping these are MAX
ratings, if it was a higher centipoise number it would then go into the 25W Viscosity Grade. So 20W oil when @ -10 Deg C Cranks oil at
4500 centipoise, which is 0.04500 grams per cubic centimeter and moves @ 0.04500 centimeters squared per second.

50 Rating is 16.3 Centistokes MIN @ 100 Deg C and 21.9 Centistokes MAX @ 100 DEG c. so the oil flow will be between 0.163 & 0.219
Centimeters squared per second @ 100 Deg C, outside of this range then will cause oil shear or stress.

So to those people running 60 Rating oils it will be 21.9 Centistokes MIN & 26.1 Centistokes max so the oil flow @ 100Deg C will be
greater.

Also the Different winter grades remain relitivly the same measurements, just the temperature drops so the viscosity that 15w achieves
while cranking @ -15 Deg is the same as 10w but while cranking at -20 Deg C, so this may mean different centipoise figures above these
temperatures So a Low W number may aid in cold starting even though the oil is not at negative whatever..

0W -30C Crank -40C Pumping
5W -25C Crank -35C Pumping
10W -20C Crank -30C Pumping
15W -15C Crank -25C Pumping
20W -10C Crank -20C Pumping
25W -5C Crank -15C Pumping

Hot Ratings MIN MAX @ 100 Deg C
20 5.6 9.3
30 9.3 12.5
40 12.5 16.3
50 16.3 21.9
60 21.9 26.1

Here's a handy calculator
Calculator

I always thought that 20 w 50 meant that when cold the oil flowed like a 20 weighted oil and when hot it had the characters of a 50 weighted oil.
That way the oil would have a consistant flow irrespective of engine temp.

clear as mud

vn5000 wrote:I always thought that 20 w 50 meant that when cold the oil flowed like a 20 weighted oil and when hot it had the characters of a 50 weighted oil.
That way the oil would have a consistant flow irrespective of engine temp.

Yeh that's right, Viscosity etc changes with Temperature, so once running @ 100 Deg c it's operating at the same rating/flow etc etc as a 50 weight oil but between -20 degc and 100c the flow, pumping surface area etc would all be different.

I guess the point of my post is there's people going and running 5W-30 in an engine rated for 20w-50, the components and clearances call for a speced oil for a reason this helps spell out why. Also ive found some oil companies list there oil specs in detail, like mobil and castrol, this way you can check the tech data for yourself, nut out which oil is actually better, as one oil labeled 20w to the next oil labeled 20w could be vastly different in the actual pumping/cranking/surface area etc properties.

Good read There is a heap of science with oils. I want to actually get an oil sample done one day - but the results might scare me, prob better off not knowing

yeh i get some done now and again on my commo, pretty good results for 270,000km ecotec, have a mate that works for CAT so get free samples done There's certainly much more in it than ill ever understand :/

As the castrol adds used to say "OILS AINT OILS "

while we are on this subject i thought i might aswell post up the oil sample i got done on the gemini oil compared to a new sample.

in my case i knew there was engine wear and water in the oil (from methanol) but i was more interested to see exactly what the results said.

cool, what oil is that?

20w-50