Exactly How Does Multi Viscosity Oil Work?

[3RRL]

I've always wanted to ask this question about multi viscosity oils...for example 5w40 oil. I've read where at colder starting temps it's easier because the 5w viscosity and then when it heats up pretty hot you still get protection from the 40w viscosity.

So how exactly does this work ... the principle of it?

In addition to the question above, when an engine seems to run better on multi viscosity oil (aside from additive packages in oil) depending what temperature the engine is at would the oil be at say 36w? Can it do that?
In other words, does it become a certain viscosity depending on temperature? If so, how does it know to do that?
Thanks,

 

[DieselPower]

"Multi-viscosity" doesn't refer to any changes in the oil's thickness caused by changes in temperature. It is a simply a rating of the relative viscosity of an oil compared to other oils -- which could be thinner or thicker.

Here's a fairly basic description of a Multi-Viscosity oil from BobIsTheOilGuy.

BMCNO Motor Oil 101

MULTI-VISCOSITY OILS
"Multi viscosity oils have polymers 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.

Multi-viscosity oils are one of the great improvements in oils, but they should be chosen wisely. Always use a multi-grade with the narrowest span of viscosity that is appropriate for the temperatures you are going to encounter. In the winter, base your decision on the lowest temperature you will encounter; in the summer, the highest temperature you expect.

10W-40 and 5W-30 require a lot of polymers (synthetics excluded) to achieve that range. The polymers can shear and burn, forming deposits that can cause ring sticking and other problems. This has caused problems in diesel engines, but fewer polymers are better for all engines. The wide viscosity range oils, in general, are more prone to viscosity and thermal breakdown due to the high polymer content. It is the oil that lubricates, not the additives. Oils that can do their job with the fewest additives are the best. Follow your manufacturer's recommendations as to which weights are appropriate for your vehicle. "

 

[CurlyDave]

DieselPower's post is pretty much the same explanation I understand, with the exception of the last paragraph.

The wider range multi-viscosity oils are better lubricants than the lower range and single viscosity oils. This is due to something called the "Weisenberg Effect".

Any fluid with polymers added will exhibit some very strange behaviors at times. The most common example of this can be seen when your wife mixes a cake. The cake mix actually climbs up the electric mixer heads. We have all seen this so often, we don't even recognize it as very unusual behavior for a fluid. To get a comparison, put a clean mixer in a bowl of plain water and turn it on. The water will go down in the center as it swirls around the mixer heads. This is "normal" fluid behavior.

For a bunch of complex reasons, which require tensor mathematics to understand, multi-viscosity oils cause forces to develop in the lubricating film which actually separates bearing surfaces, significantly reducing wear. These forces arise from the same phenomina which cause the cake mix to climb up the mixing heads.

Most modern engines, both gas and diesel, specify multi-viscosity oils because of this reduced wear effect.

The days of polymer additives "burning" and decomposing are far behind us.

CurlyDave

 

[3RRL]

Thank you for the great explanations and the link. I will keep it for future reference.

 

[RalphVa]

That business about selecting the most narrow viscosity range needed just does not apply today to synthetic oils because they have little to no additives that tended to do strange things.

Oil viscosity changes with temperature just as does almost any natural substance that is liquid/semi-liquid. In the case of oil, we measure this tendency with something called viscosity index. Some natural materials, like wax, have a very high natural viscosity index and would probably be something like a 5w or 0wxx type oil IF it didn't freeze and become wax. What people have done in making synthetic oil is to make an oil with wax-like viscosity index, but it doesn't freeze and become wax. Actually, what they do is make long chain paraffinic molecules with some occasional side chains that keep the chains from coiling up and freezing at normal atmospheric temperature. Such oils will still be fluid down to near -40 temperature and will not get as viscous at low temperatures as regular dino oils.

If you want the very best engine protection and live in a place where it gets fairly cold in the winter, you should buy a synthetic 0wxx or 5wxx oil and use it year round. The "xx" that you buy depends on what the engine manufacture recommends. Nowadays, many of them are recommending 5w30 in diesel engines and 5w20 in gas engines. If it gets very cold, 0w30 and 0w20 would be even better.

My old Benz recommends 10w40 for it, and back in 1983 that was about the maximum available vis range available. I'm using 5w40 in it now. In my tractor, since JD recommends a 5w30, that's what I'm using. However, if I had it in Vermont, I'd put 0w30 in it.

Ralph