Difference between 15w and 20w on API chart

Paul, I believe you are getting the charts confused. A 15w and 20w would represent the cold flow indicators of a fluid, hence the W after the numeral.

A 15W will show 7000cP's @ -20ºC.
A 20W will show 9500cP's @ -15ºC.

These are static viscosities.

If you are speaking of the hot temperature viscosity then a 20 ranges between 5.6 to 9.29cSt's at 100ºC. I don't know what a hot temperature 15 grade is spec'd at.

If I have misread your posting then please just dismiss my information.

Johnny, You make a good point, the 15w pumps better at cold temperatures. However, I would expect that, just as a 5w and 10w are consecutive on the viscosity range (lost my chart or I would post the numbers for example), so would be 15 and 20w, but both 15 and 20w show the same range of 5.6 to 9.29. Well, maybe they are both essentially the same viscosity range and the only difference is cold pumping.

I see what you are saying now Paul. The numbers that you're talking about are for the winter ratings of these fluids. At hot temps they are the same viscosity (100ºC). At cold temps they appear very similar yet the 15w exhibits less resistance to flow as you would naturally conclude.

What is interesting is that on the SAE J300 chart there is a 20w viscosity and a regular 20. The difference of course is the temperatures that they're tested at.

I remember seeing oils on the shelf that were 20W-20 and I used to wonder what the difference was between that and a straight 20 grade.

It is all about the flow characteristics at different temperatures.

So while there could be a 20W-20 that had a cold temp property of 9500cP's @-15ºc, the hot end of the scale could range from 5.6 cSt's up to 9.29cSt's which would practically give you a 20w-30 if you found one blended at the extreme limits of the chart.

But back to the 15w and 20w, when tested for cold flow the 15w returns a lower viscosity at a even colder test temperature so the difference between the two is probably greater than you would assume in the extreme cold.

It 's a very interesting observation none the less.

Great Johnny. I have tried to get through on this at the oil guy site and it just didn't work. Maybe I just wasn't expressing myself well. Anyway, I have a viscosity chart at home that shows the progression through 15w then 20w, but it does not line up with the viscosity range for the API table. I will try to post the data this evening for your edification.

As for 20w20 I recently read an article that said an oil rated "SAE 20" acts like a 20 wt at 100C (no guarentee apparently what it acts like at 0C) but that a "20w20" acts like a 20 wt oil at 100C and like a 20 wt oil at zero or whatever the w temp rating is. So I guess there is difference between a 20 and a 20w20.

An absurdity of the '70s was they had oils rated like this: 10w20w30. Seems an impossibility.

I found the reference, right here on Noria.com from the July 2001 issue of Machinery Lubrication, in "ISO Viscosity Grades":
http://www.machinerylubrication.com/Backup/200107/viscosity_table_2.gif

Look at the table and you will see that SAE 10W ranges approximately from 4.8 to 5.8 cSt @100C on the Kinematic Viscosity. That would put 15w in the upper half of the API 10W range, whereas the API chart has it in the 20W range. Not sure what is going on here.

The API chart also shows the same 100C viscosity range for the pairs 0W/5W and 25w/30.

In the old days there was 10w and 20w. It was truckers that started a thought toward a robust cold startable oil. The 15w is exactly 1/2 way between 10 and 20 wt by definition. My speculation is this, take a mono 30wt. like rotella that flows well cold for a 30 and add some VII"s to it. This flows like a 20 cold and with vii's like a 40 hot, add some more and now you have a 20w50. To my mind use a 20w40/50/60 like a mono 30 wt. Now a 15w40, flows better, so a 20wt. base oil before it is modded up. So in this case treat a 15w40/50 like a 20w20 mono, or at least use as a thought model.