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Oils viscosity ratings

So ive been doing some research on motor oil and what all the numbers I see actually mean, and this is what I have read about multi-viscosity motor oils. lets take 5w-30 as an example. 5 is its viscosity rating at cold tempertures (hence the W for “winter”) and 30 is its rating at 212 degrees F. The lower the number the more easily the oil flows, correct? BUT oil thins as it get warm, isnt this also correct? So why is the rating number going UP as the oil gets warm?

A 5W30 when warm will have the same viscosity as a straight 30 weight does warm. It’s done with “viscosity modifiers{”, microscopic polymer coils that unwind as they heat up with the oil and affect its viscosity.

Visit for a good primer.

Those numbers are a viscosity index, not the oil’s viscosity. 5W30 does not get thicker when it is hot, it just doesn’t thin as much as straight SAE 5 weight oil does. That’s because cold 5W or even 0W oil is thicker than hot 30 weight oil.

Viscosity is measured in Saybolt Universal Seconds (SUS) or in centipoises (metric).

Take a look at this recent discussion, there are examples that explain this:

so from that link i gathered “5w-30 oil will act like a 5 weight oil when cold, and like a 30 weight oil once warm”

BUT isnt an 30 thicker then an 5? oil thins as it gets warm so i dont get how that makes sense.

everyone keeps saying, “when cold it will act like a 5 but will not thin as much as a 30” how can it thin to a 30 when a SAE30 oil is thicker?


Have you seen yet?

yes i was reading it today, i didnt like that website very much. But according to his viscosity chart, Is it showing that cold and hot go by completely diferent scales? So in a 5w-30 for example, when cold its going to run ~92 SUS (~18 cSt) and when warm its going to run ~63 SUS (~11cSt)?

OR another example

an SAE30 would run ~480 SUS (~100cSt) when cold but ~63 SUS (~11cSt) when warm?

i think im finally getting it! the fact that cold and hot each have their own scale really clears it up now :slight_smile:

So (hypothetically speaking) if an oil were a 5w-50, there would hardly be a change in its viscosity

Here’s the (made up) example I used there:

Oil weight/‘cold’ viscosity/‘hot’ viscosity
10w / 20 / 2
30w / 100 / 10
10w-30 / 20 / 10

See, they all get thinner when they heat up, but the 10W-30 decreases less (50%) than the straight weight oils (90%).

“BUT isnt an 30 thicker then an 5? oil thins as it gets warm so i dont get how that makes sense.”

Some things get a lot thinner when they get cooler (think of butter coming out of the fridge and when it warms up a little it is thin. Now try that same thing with wheel grease. It will be thinner when it just comes out of the fridge than the butter and when it warms up it will be thicker than the butter at the same temperature.

10X30  Will be as thick as 30 weight oil (at the same hot temperature) when it is hot and it will be the same as a 10W oil when both are equally cold. (well as I recall the test temperatures are 0 and 100 Celsius. 

10x30 will be thinner when it is hot than when it is cold, but just not as much difference a than the single weight oils.

So (hypothetically speaking) if an oil were a 5w-50, there
would hardly be a change in its viscosity

A 5w-50 weight oil will flow and pour just like a straight 5 weight oil at cold temps (where I recall cold temps being defined as 0 degrees C).

That same 5w-50 weight oil will flow and pour just like a straight 50 weight oil at hot temps (where I believe hot temps were defined to be 100 degrees C).

The late Tom McCahill, who wrote the automotive column for Mechanix Illustrated, didn’t think much of multi-viscosity motor oils. In his book, “What You Should Know About Cars”, he claimed that 10W-30 motor oils were really a lousy number 10 oil and a lousy number 30 oil. He thought the real purpose of the multi-viscosity oil was so that dealers wouldn’t have to stock as many different oil weights on the shelves.

Now I know that oils have improved since McCahill’s time. However, I was using 10W-40 in my 1978 Oldsmobile which the owner’s manual said I could use. I had a real problem with pre-ignition (spark knock). I was constantly pouring Casite Motor Tune-up through the carburetor to remove the carbon deposits. On car talk, someone called in with the same problem. Click and Clack suggested that the problem might be the 10W-40 oil that the caller was using. Apparently, the polymer coils could cause a carbon build-up on the piston tops. I switched to 10W-30 (also allowed by the owner’s manual) and have had no more spark knock problems with the Oldsmobile.

I don’t agree with McCahill’s position on multi-viscosity motor oil, but I really miss his columns in Mechanix Illustrated.

I doubt that it’s linear. With enough viscosity modifiers to creat a 5w-50 oil the modifiers would probably become dominant. At high temperatures the stuff would probably turn to a polymer sludge.

I’ve not done the research, I’m just guessin’ here.

well my last message was rhetorical, but thats ok. If you dont know what it means then refer to the “viscosity chart” on bobs oil website. And yes, a 5w-50 DOES have very little change in viscosity between hot and cold.

Mountainbike is correct; over the 0F to 212F the viscosity decrease is linear, but much outside those ranges we have a jell at the bottom and a polymer sludge at the top.

Motor oil viscosity is tested at 0F and 212F. A “W” means the oil will flow at 0 degrees. Some #20 oils can meet this requirement and some can’t…

VIS, viscosity index improver, the additive used to convert #5 oil to 10W-30 oil, leaves heavy deposits when burned resulting in the detonation problems mentioned above. If an engine demands a “low ash” oil, that oil will always be a single grade oil. All 2-stroke engines and some turbo-diesels require low ash oil…

Those ratings mean that the oil tries to imitate some of the properties of the two viscosities that are listed. It doesn’t mean the oil is 5 weight when cold and 30 weight when hot. In truth straight 10 weight will always be thinner than 5W30 and straight 30 will always be thicker than 5W30; especially in the cold and especially in the heat.

The ratings are only trying to express what ranges of oil that the multiweight oil is trying to imitate. In other words they are claiming that the oil won’t be too thick to use in Winter cold or too thin to use in Summer heat.

Wow! some people need to research this topic a little better…

Wow! some people need to research this topic a little better…

Can you explain?