Whats it all mean?

[Jeffrey Behr]

Most of this is easy; bare with me.

The numbers denote the S(ociety of)A(utomotive)E(ngineers) viscosity (thickness) classes. Each class is defined as a range and the details can be seen here. This characteristic is NOT weight--all engine oils weigh virtually the same--but most of the industry and even the oilfreaks who populate this board still misuse that term. The higher the number the thicker the oil.

The 'W' means Winter or Winterized. It's an old (and probably unnecessary) term denoting oil that will not evaporate or burn off as quickly as the lighter oils that aren't 'W' rated.

The 2 numbers used for what's called a multigrade oil are a little more complicated. The first (or only) number is the viscosity range measured at 40dC. The 2nd number denotes the viscosity class the oil has at 100dC, which approximates operating temperature.

All oils thin as temperature increases; a multigrade oil thins less than a single-grade oil. Look at the chart again and the 2 left-hand 'Kinetic Viscosities' columns. The 2nd number of a, say, 10W-30 oil means that it has thinned (at 100dC) only to the range associated with the industry-standard 30-vis. oil.

Multigrade oils do a better job of lubricating cold engines at startup and hot engines when operating.

Hope this helped.

One more term. Viscosity index or VI is a measure of an oil's resistance to thinning at high teperatures. The higher the number, the less it thins, so multigrade oils by definition have higher VIs.

 

[WVSpaceman]

so let me see if i got this right. in say a 5w20, at operating temp its going to thin to 5, and at a set cold temp its going to be 20?

 

[n8wvi]

Nope, in your example, a 5w20 at 0F will behave like a straight 5 weight oil will at 0F. At 212F, it will behave like a straight 20 weight oil will at 212F.

 

[Mike_dup1]

In plain english.....

Found this on the API site, GO SEE FOR YOURSELF.

quote:

---

The low-temperature viscosity (the first number, 5W in a 5W-30 oil) indicates how quickly an engine will crank in winter and how well the oil will flow to lubricate critical engine parts at low temperatures. The lower the number the more easily the engine will start in cold weather.

The high-temperature viscosity (the second number, 30 in a 5W-30 oil) provides thickness, or body, for good lubrication at operating temperatures.

A multigrade oil (for example, SAE 5W-30) provides good flow capability for cold weather but still retains thickness for high-temperature lubrication.

---

[ February 03, 2004, 01:51 PM: Message edited by: Mike ]

 

[WVSpaceman]

thats the opposite of what the others were saying, or am i totally confused now? 5 would be thinner than 20, since gear oil is 90

 

[haley10]

Just think of the top number as thickness at operating temperature. Think of the bottom number as a "relative" Winter cold crank ability.

A 5W-30 will be 30 wt. at operating temps just like a 10W-30, but it will cold start easier if the temperature is zero outside.

These numbers have a wide range and are not absolute either.

 

[Eiron]

Forget about gear oils for right now, 'cuz their viscosities are measured at colder temps than motor oils.

 

[TC]

A good, basic primer on motor oil:
http://popularmechanics.com/automotive/auto_technology/2002/8/motor_oil/index.phtml

 

[turbochem]

quote:

---

thats the opposite of what the others were saying, or am i totally confused now? 5 would be thinner than 20, since gear oil is 90

---

The "5" and "20" are not directly comparable because they are determined at different temps.

 

[WVSpaceman]

OK, im new to this forum, so forgive me. im sure this questions been asked many times before, and ive asked it many times without an explaination.

What is the meaning of the #'s and the "W" in the oils. ie: 5W20, 20W50, 75W90, 75W145, and so on?

I know the second # is the oils weight, but then i could be wrong.

Could someone explain it all to me?

 

[TallPaul]

Maybe this'll help. From the August 28, 2002, Lube-Tips newsletter at Noria.com:

"From "The Practical Handbook of Machinery Lubrication":

Multigrade oils are made by blending a low-viscosity oil with special additives called Viscosity Index Improvers. For example, when these polymer additives are blended in the correct proportion with an SAE 15W oil, the oil flows like an SAE 15W oil at low temperatures and like an SAE 40 oil at high temperatures. The result is an SAE 15W/40 oil that will provide wide protection over an extended temperature range. "

 

[haley10]

The top number is the oil's kinematic viscosity when heated to 100C/212F. I think of this as a "real world" thickness at near operating temperature.

All oil will thicken when it's colder. The bottom number is a measure of an oil's cold cranking ability at a given temperature. This is usually -25C for 10W and -30c for 5W. This is done by actual test using a mini rotary viscometer. It's really a relative indicator of cold cranking performance.

 

[TSoA]

My impression is that the W rating has nothing to do with viscosity at a given temp, rather it simply indicates at which (low) temp it can pass a cold-crank test. The second part of the indicated grade is self-explainatory.

 

[XS650]

quote:

---

Originally posted by TSoA:
My impression is that the W rating has nothing to do with viscosity at a given temp, rather it simply indicates at which (low) temp it can pass a cold-crank test. The second part of the indicated grade is self-explainatory.

---

The tests are actually pretty good viscosity tests. The low temp tests consist of cranking and pumpability testrs. They are effectively real world viscosity tests. Viscosity varies considerable depending on shear stress and shear rate and those tests evaluate the oil under more realistic conditions than a simple squirt test.