multi viscosity oil question

[maxamillion]

1.) Ok say you have a 10w-40 oil. 10w stands for the winter/start-up viscosity, 40 for engine operating viscosity. Since oil thins out when hot and thickens when cold, How can the the oil be rated 10 viscosity (thin) at colder engine temps then raise to a 40 viscosity (thicker) at higher engine temps?

It seems like the # rating should be flopped around. Could someone w/ good oil knowledge explain this to me. I think Im missing something simple here.

2.) And same w/ a straight weight oil. Say you have a straight 30 weight. The viscosity is obviously higher(thicker) at engine start up, then as the engine reaches operating temps the oil thins out and the viscosity lowers (gets thinner). So why is it considered a straight weight when the viscosity changes?

Thanks-Max

[Ace Sixx]

http://www.micapeak.com/info/oiled.html

[wilkin250r]

You are confusing rating with actual performance.

The RATING is a measurement of viscosity at a specific temperature, usually about 20 degrees C.

The actual PERFORMANCE is the issue you are thinking of. Yes, a straight-weight oil will change viscosity over temperature, it will get thinner as it heats up. But it's predictable, exactly as calculated. The performance changes, but the rating does not.


Viscosity is measured in centipoise (cP), and as you know, it will change with temperature. Let's put together a little table, and it will answer your first question.

Let's say:

At 0 degrees C:

10W oil - 500cP
40W oil - 2000cP
XXX oil - 500cP

At 150 degrees C (normal engine operating temp).

10W oil - 80cP
40W oil - 250cP
XXX oil - 250cP


Look at XXX oil. Even though it does get thinner as it heats up, it's acting like a thin 10W oil at low temps, and a 40W oil at high temps. It's a multi-viscosity oil, standard 10W-40.

[maxamillion]

OK I think im slowly getting a better understanding here. The rating is not saying the 10W oil thickens up at operating temps, it just implies that it performs like a straight 40W would at operating temps, correct?

This is off that posted site. It seems to help a little.

"""Multi viscosity oils work like this: Polymers are added to a light base(5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot."""

Thanks and if anyone has something else to add feel free.