# Does motor oil get more viscous when heated & what is the meaning of the SAE motor oil code?

The motor oil in my 2004 Pontiac Vibe is 5W-30.

My (incorrect) understanding of this SAE nomenclature is that 5W means "Viscosity index of 5 in the winter" (or when really cold - like $$\pu{-30 ^\circ F}$$) and "Viscosity index of 30 at operating temperature" (or when the car has been running for a little while).

So how is this accomplished? I usually think of something like butter or coconut oil that solidifies at a lower temperatures and becomes fluid again as heated. To my understanding oil has a higher viscosity at lower temperatures (generally speaking).

Everyone keeps talking about "additives" that help accomplish this feat in "multi-grade" oil. What are these additives? Generally, how do they work? I'm no chemist and neither are these auto mechanics, so I figured this might be the right place to ask.

Since my understanding of the SAE motor oil code is incorrect, illumination on its exact meaning would be helpful.

A full blown chem answer and an "explain it like I'm 5" would be most appreciated :) I never studied chemistry past high school, so be easy on me!

The numbers in the code have nothing to do with viscosity index. However, your understanding of 'W' for winter is correct. The real code for your car is actually SAE 5W-30 where SAE stands for the Society of Automotive Engineers, which has established a multi-grade numerical code system for grading motor oils according to their viscosity characteristics. I said viscosity characteristics, not viscosity index. According to Wikipedia:

The original viscosity grades were all mono-grades, e.g., a typical engine oil was a SAE 30. This is because as all oils thin when heated, so to get the right film thickness at operating temperatures, oil manufacturers needed to start with a thick oil. This meant that in cold weather it would be difficult to start the engine as the oil was too thick to crank. However, oil additive technology was introduced that allowed oils to thin more slowly (i.e. to retain a higher viscosity index); this allowed selection of a thinner oil to start with, e.g., "SAE 15W-30", a product that acts like an SAE 15 at cold temperatures (15W for winter) and like an SAE 30 at $$\pu{100 ^\circ C}$$ ($$\pu{212 ^\circ F}$$).

viscosity characteristics: Kinematic viscosity is graded by measuring the time it takes for a standard amount of oil to flow through a standard orifice at standard temperatures. The longer it takes, the higher the viscosity and thus the higher the SAE code. Larger numbers in the code are thicker.

The company called Anton Paar has analyzed these SAE coded motor oil for the parameters, Kinematic viscosity and oil density versus elevating temperatures. I have included some of graphs including SAE 5W-40, but failed to find the most common SAE 5W-30:

Finally, for your information, Viscosity Index is a dimensionless number that represents how the viscosity of a hydraulic fluid changes with temperature. The greater the viscosity index, the smaller the change in fluid viscosity for a given change in temperature, and vice versa. Synthetic oils usually have a higher viscosity index than do mineral oils. (Mobile Hydraulic Tips). The following graph illustrate how viscosity index influence the change of viscosity with temperature (Ref.1):

It is also nice to know how change of viscosity index by modifiers affect mono-grade oil to better multi-grade oil:

References:

1. Tiago Cousseau, “Film Thickness and Friction in Grease Lubricated Contacts: Application to Rolling Bearing Toque Loss,” PhD Thesis; University of Porto (Universidade do Porto), Porto, Portugal, 2013.
• This is a top notch answer! You really do nice work, Mathew! – Ed V May 24 '20 at 18:37
• @Maurice: the answer to that is: none, because the viscosity of motor oils does not increase with temperature. – cbeleites unhappy with SX May 24 '20 at 19:29
• @Maurice I was thinking the same. Very well researched answer, and extremely interesting, but it doesn't address the crux of the question; how does it accomplish this? cbeleitesunhappywithSX points out that the viscosity in fact does not increase with temperature; I am hopeful that someone can help me with this quandry! – Shrout1 May 25 '20 at 14:21
• @Shrout1: I did not concentrate on chemistry part because you insist that you do not have chemistry background. Beside, these facts of how they do that are not published in order to keep them for themselves. Specifically, synthetic oils, which behave much better than natural oil are patented (e.g., Mobil 1 5W 30).. – Mathew Mahindaratne May 25 '20 at 14:43
• @Shrout1: I see your point. Keep working on it. And, it won't hurt if you post another question concentrating on that regard. So, you'd get more help from others. Good luck.:-) – Mathew Mahindaratne May 25 '20 at 15:28

So this is a gross oversimplification of this issue, however it is a very high level overview of the mechanism at work. From a blog that shell oil put out:

Mineral multigrades use a light weight mineral oil (the same as a light weight monograde oil), but include an additive called a Viscosity Index Improver. The best way to visualise this viscosity index improver is as a long chain molecule which curl up like a ball of string when cold, but then uncurl as the temperature increases.

Thus when an oil is cold, the presence of the viscosity index improver has very little effect and the oil flows well as the base oil is a low viscosity oil. As the oil heats up, the viscosity index improver uncurls with the effect that it tends to restrict the motion of the oil, or 'thickens' it, which to some extent counteracts the decrease in viscosity of the base oil. This enables the oil to support more load than would otherwise be possible.

This blog also mentions that these VII's break down when sheared (so like... in an engine) and lose their effectiveness. Time for an oil change!

Self answering as it has been a number of months; I'd still be extremely interested in the chemistry behind these VII's though I'd imagine that is incredibly complex; hydrocarbons usually are if I'm not mistaken. It's also likely proprietary, or at least the latest iterations probably are.