What Do Oil Grades Mean?

[Vlad Soare]

Hi,

In know how oil grades work - the lower number is the viscosity at low temperatures, the higher number the viscosity at working temperatures, lower on the left side is good in winter, higher on the right side is good in extreme heat or on the track, and so on. I know the general theory.
But what exactly do those numbers mean? In a 5W30 oil, 5 is the viscosity at low temperatures. OK, I get that, but what does that mean? Five what? Pascal*seconds? Poises? Centistokes? Or what? And what exactly does "low" temperatures mean? How low? Try as I might, I couldn't find a straight answer, something like this: "an oil with a rating of 5W30 will have a viscosity of 5 <insert measuring unit here> at xxx degrees Centigrade, and 30 <insert measuring unit here> at yyy degrees Centigrade". All I could find were graphs showing the ranges of temperatures over which each grade is meant to work well. But those don't answer my question.
We all know that lower is thinner, higher is thicker, lower is better in winter, blah blah blah. But that's just a superficial concept meant to describe the basic idea in layman's terms. What's the exact definition of those numbers?

Thank you.

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[Strokerswild]

The SAE document J300 defines the viscometrics related to these grades, cP and cSt both come into play. Kinematic vicosity 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 are thicker, the 'grades' are just a series of steps related to flow.

 

[Vlad Soare]

OK, I get that - higher numbers mean higher viscosity. And I get how the viscosity is measured - how much time it takes for the oil to flow at a given temperature. Fine. But what exactly is 'standard' temperature? How does a certain SAE grade translate into "viscosity xx <whatever measuring unit> at yy degrees"?
And another thing: a 5W30 has a viscosity of 5 (whatever those are) when cold (whatever 'cold' means), and a viscosity of 30 (whatever those are) when warm (whatever 'warm' means). But why do we want different viscosities at different temperatures? Wouldn't an oil with a constant viscosity over the entire range of temperatures be more desirable? Wouldn't a 30W30 be perfect?
I searched for document J300, but they want 83 dollars for it.

 

[Strokerswild]

https://selectsynthetics.com/part-2--oil-viscosity-grades-.html

 

[Vlad Soare]

That seems to be exactly what I was looking for. I'll give it a read and see if it answers all my questions.
Thanks.

 

[Vlad Soare]

Indeed, that's the kind of information I was looking for. Things are starting to make sense now.

Now the cold part is clear. There's no such thing as "too thin". The lower the left digit can be, the better. All other things being equal, there's no reason whatsoever to choose 5W instead of 0W. If, say, the manufacturer recommended 5W20, then using 0W20 would be even better.
But for the warm part we still need to know three more things:
1. What's the optimum viscosity? If, hypothetically speaking, someone could make an oil with a perfectly constant viscosity at any temperature, under any circumstances, what would we want that viscosity to be?
2. What's the actual temperature, or temperature range, of the oil in a warmed-up engine?
3. What's the viscosity of each SAE grade at various other temperatures besides 100 degress Centigrade?

The answer to no. 1, according to the article, appears to be 10 cSt. So the closer we can get to 10 cSt, the better. What's not clear is how far we can afford to deviate from 10 cSt without causing damage to the engine. In other words, if 10 cSt is the optimum viscosity, what will the not-necessarily-optimum-but-still-acceptable viscosity range be?

The answer to no. 2 will probably depend a lot on each particular engine, so I guess it should make the object of another topic.

The answer to no. 3 may also depend on each particular oil brand and type, so that a straight, universally valid answer may not be possible. Two SAE 30 oils will both have a viscosity of around 10 cSt, give or take, at 100 degrees Centigrade, but at other temperatures they may be quite different from one another. Or maybe not? Does the SAE standard also cover temperatures other than 100 degrees?

So the answer to the question "which oil grade is best" appears to be like this:
- to the left ot the W, the lowest number you can find
- to the right of the W, the one that's closest to 10 cSt at the temperature your engine happens to run at.

 

[Shifting_Gears]

Good info.

Very subjective when you start looking at performance oriented engines, engine tolerances and additives in different branded oils that may effect the ability to resist shearing, degradation of the oil’s weight rating and ability to flow properly under “stress”.

Gone are the days of “thicker is better” in the sense of outright protection.

 

[CrashOverride]

I'm not a petroleum engineer, although we do have one on here, so it's possible he may comment and/or correct me if I'm wrong. Oils get their viscosity range using viscosity improvers (VIs), which are part of the additive package that is added to base oils. In general, dino oils need more VIs to hit a range than synthetics. These VIs are one of the things that "wears" in the oil. They are also responsible for some of the ash content in the oil. Generally speaking, the lower the VI's the better. That's where using a better base stock (read: best grades of synthetic) really helps. So, going with the widest range all but guarantees you are going to have more VIs in the oil which isn't the best.

The viscosity question is more complicated. The optimal viscosity is the lowest number that can maintain a film thickness sized by the clearances in the bearings (Cam, crank, rod). Things that affect that thickness are shock load (combustion events), heat, and viscosity shear over time. If the bearing clearance is 0.001, and you run a straight SAE 30 oil through it, at normal engine operating temperature at normal engine speed/load, with new oil, let's say it maintains that clearance with no issues at all.

Now, same oil but this time with 5000 miles on it. Viscosity has sheared to SAE 20 weight because of VI decomposition. And now let's put the car on a track, so the engine is hotter than normal, engine speeds are higher than normal, and the pump is cavitating from the oil sloshing around in a turn. In this scenario, the clearance cannot be maintained and the oil film cannot support the 0.001 clearance, so despite still having 0.0009 clearance, a piece of sand that made it's way through the air filter and got absorbed into the oil having a grain size of 0.00095 hits that bearing clearance, because the oil film isn't thick enough, the sand will scar the bearing, journal, or both. (I pulled these numbers out of thin air, they are just used to illustrate)

So viscosity selection is quite complicated. Oil is also a coolant, and if the oil cannot pump fast enough, then the oil cannot wick away the heat fast enough and that can cause problems. Also, being a higher viscosity means that it is harder to pump, and that directly transfers to the oil pump gears, which in a lot of cars (This included) can cause issues at WOT because the gears are made of powdered metal and have a tendency to crack under high speed/high load.

Now, conspiracy theory or not, a lighter weight (lower viscosity) oil is going to be easier to pump. Less pumping energy means less torque loss, less torque loss means less HP loss. With more HP available, less fuel is needed to overcome air resistance, and the end result is better fuel economy and less emissions. I'm sure the amount is fairly low, but those efficiency gains cost almost nothing since changing oil viscosites is extremely cheap (* Note, I don't actually know this from a manufacturing standpoint, but as a dumb old consumer, the price between grades sold on the shelf is often negligible).

How do you know what oil is best for you/your car? send your oil to blackstone labs. Then drive exactly the same in the same weather, same traffic, same speeds, for 5000 miles, changing only the oil viscosity. Then, see which viscosity gives you the lowest particulate matter in the metals used in bearings. Easier said than done of course, which is why "best guess" applies here.

All this technobabble aside, in most cases, if you use a bottle of oil found on the shelf at wal-mart, and you change it frequently enough, you are going to be fine. (I said wal-mart because they aren't going to sell break-in oils and other very thick viscosity racing oils like Autozone et al. will sell). I bought a car from my neighbor because it stopped running and just wasn't worth it for her to fix. It had over 200,000 miles on it and she didn't change the oil, but instead the thing leaked more oil than the exxon valdez. When I pulled the valve cover off, the engine looked brand new. It looked so good, I brought her over (And her mechanical engineering husband) to show them just how clean it was.

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