Still trying to understand this?

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Mar 16, 2003
take a 10w30 oil, the 10w is the weight/thickness when cold. The 30 is the oil's weight and thickness when hot, right? Well there are a lot of threads that talk about how an oil thins out when hot. Shouldn't an oil go form a 10 weight to a 30 weight as it warms and get thicker? If so then why do I keep hearing folks talk about how an oil is thinning as it warms? I should know this by now!
How about thinking about it this way:

1) The oil thins as it gets hot (fact) (It's not a total linear relationship, well maybe it is on a log scale, but DON'T worry about that)

2) The oil is thicker at XX (40 or whatever less than 100)°C than at 100°c (fact)

3) The 10W is viscosity is measured at a low temp and recorded. The 30 is measured at 100° C and recorded. But the two really have nothing to do with each other as far as the measuring goes...they are specification categories oils are put into, so to speak.
My understanding is that the "W"inter classification is totally unrelated to the viscosity number (2nd) numerically. The "W" is representative of how the oil performs in a cold cranking engine. Oil thins as it heats up & thickens as it cools. Oils that thicken less at cold temperatures are rated with lower "W" numbers.
think of the number 5, 30 whatever, as not an actually thickness, but a reference oil. So the 5w30 is the same thickness when cold as a 5 weight oil, and the same thickness as a 30 weight oil when hot. So basically it doesn't thin as much as a straight weight oil.

Yeah. I'm sorry but I have a hard time understanding this. It is thicker cold and thinner hot so why isn't it labled 30w10? Where do they get off saying it is a 30w hot when it is indeed thinner hot? It would make more sence to us "slow" folks!

EDIT. This was in responce to Pablo.
So the 10w is not a thickness? If it is thinner when hot why do they use a thicker number for it?
Okay- Think of the numbers like this. At 0 degrees the oil will behave like an SAE 10 weight oil does at 0 degrees. At 100 degrees the oil will act like a 30 weight does at 100 degrees. Essentially a split weight oil is designed to act like a thinner weight at lower temps and thin out less as it is heated so that it acts as a heavier weight at higher temps. Okay?
No the number is not a measurement it is a reference oil.

Here maybe this is a better example. Hypothetical only, not exact numbers. Cst is a measure of an oils thickness.

Imagine a 5 weight oil, a 30 weight oil and a 5w30 oil.

At 40deg Celsius the 5 is 40 Cst, the 30 is 100 Cst, then the 5w30 would be 40 Cst.

At 100deg Celsius the 5 is 4 Cst, the 30 is 11 Cst then the 5w30 would be 11 Cst.

As you can see all the oils thinned out, but the 5w30 thinned less so than the others. That's because it's a multiviscosity oil. It is confusing because it means a multiviscosity oil changes thickness less than a single viscosity.

Sorry if I've botched any terms, just trying to make things clearer.
Slightly off topic:

I understand that oil viscosities are normally measured at 40°C and 100°C. I was told that the "W" after a number indicates the viscosity was measured at 0°C...

So a 5-30 oil would behave like a 5 @ 40°C and like a 30 @ 100°C...but a 5W-30 would behave like a 5 @ 0°C and a 30 @ 100°C...correct?

Originally posted by T-Keith:
So the 5w30 is the same thickness when cold as a 5 weight oil, and the same thickness as a 30 weight oil when hot.

Not sure whether this is different than what you're saying, but I believe an SAE 20W-20 and an SAE 20 are different. The cold spec has nothing to do with the hot spec. If you wanted, you could probably make an SAE 20W-5. There's more to the W specification than just viscosity at 0 deg F. Someone please correct me if I'm wrong though.
SAE 20 oil must be minimum low-sheer-rate 100C viscosity of 5.6 cSt, but less than 9.3 cSt, and have minimum high-sheer-rate viscosity at 150C of 2.6 cP.

An SAE 20w oil must have maximium low temp cranking viscosity of 9500 cP at -15C, a maximum (with no yield stress) low temp pumping viscosity of 60,000 cP at -20C, and a minimum low-sheer-rate 100C viscosity of 5.6 cSt.

A 20w20 would meet all the above criteria.
I hate to point out the obvious, but most of the explanations on this subject are dramatically different. I have learned quite a bit here on this forum over the past several months, however, it surprises me that such a basic oil labeling quality creates so much confusion even amongst the experts.
From here and other sources, I still maintain that the two numbers ie: "5w30" are not numerically comparable. A 5w is just a 5w - it doesn't behave like a 5 weight oil at any temperature. The "W" simply means in a cold cranking simulator test it falls into one of the winter categories of 0w,5w,10w,15w, or 20w.
Thank you for all the input. I think I got it.

I'll study these explanations so I will remember this. I just hope I don't have to explain it to anyone!
I'll have them stop by BITOG and read up on it instead!
Reference Oils were used to establish the "W" ratings. This was so the general public would understand the ratings.

"W" rating requirements were based on the expected minimum performance of a oil of that SAE grade. So if you were to test a 60 year old SAE20 I expect you would 20W performance at low temps. Most modern Straight Weight Oils would be about 15W-20 due to the better basestocks employed.

The purpose of multi-viscosity rated oils is to keep you from having to change from SAE 30 in the summer to SAE 20 in the fall and SAE 10 in the winter.

The following is a oversimplification of the rating system:

1. All oils get thicker as they get colder and thinner as they get hotter. The numbers on the bottle 10W or 30 are viscosity ratings not viscosity measurements.

2. A SAE 10 would be thicker at 0&#186C than a SAE 30 at 100&#186C

3. A SAE 10W-30 is as thin as a SAE 10W-10 (Most modern SAE 10 would meet this easily) at 0&#186C and as thick as a SAE 30 at 100&#186C

4. Basically the wider the spread between the "W" rating and the normal High Temp rating the less the oils viscosity changes with temperature.

The reason we are not all driving around with 0W-30 or 10W-60 is that a multiviscosity oil is basically a thin oil with viscosity index improvers that keep the oil from thinning as much as oil heats. The draw back is they are not as durable as the basestock in most cases. Better basestocks allow for higher spreads with less or no VII. This one of the big advantages of synthetics.

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