Explain oil viscosity scales?

ZeroPapaGolf

Well-Known Member
Can someone explain how single-grade oil viscosity is measured compared to multi-grade? I understand that 15w50, for example, is 15 weight at cold temperature, but thins out like a 50 weight would as it heats up to 100C. But I'm not clear on how the single-weight scale works. For example, a 100w couldn't possibly mean 100 weight, could it?
 

rframe

pǝʇɹǝʌuı
If you look on the bottle W100 is SAE-50, W80 is SAE-40, etc.... cut the W number in half.
 

Hammertime

Well-Known Member
Oddly enough, IIRC, that doesn't always work. It works for all the popular weights in use in GA, but if you end up using a wierd viscosity, it may not be exactly half. It's just that SAE picked one set of random numbers to measure viscosity, and the FAA another (MIL SPEC I think). BTW, for the curiuous, the "W" in multigrade stands for winter. I.E. 10w30 means SAE 10 in the winter, and SAE 30 when it warms up.
 

trafficinsight

Well-Known Member
if I remember right It goes something like this:

There's this apparatus called a Seyboth Universal Viscometer. A certain amount of oil is heated to a certain temperature and then drips out of the apparatus, the number of seconds it takes to drip out is the viscosity, for example, 100 weight oil takes 100 seconds to drip out. SAE cuts the number in half. There are other ways to measure the viscosity in this day and age, I'm sure.

The W stands for "Winter" and "multi-grade" is kind of a lie. If you consider that a single grade oil (say, SAE50) becomes less viscous as temperature goes up then you could draw a curve showing this relationship. You could draw the same curve for a lighter weight single grade oil (say, SAE20). In very simple terms, multi-grade oil is simply an oil where the curve begins at the high point of the lighter weight oil, and ends at the low point of the heavier weight oil.

so 20W50 has the viscosity characteristics of SAE 20 weight oil (Seyboth 40) at colder temperatures, and SAE50 weight oil (Seyboth 100) at higher temperatures.

Clear as crude? ;)
 

z987k

Well-Known Member
if I remember right It goes something like this:

There's this apparatus called a Seyboth Universal Viscometer. A certain amount of oil is heated to a certain temperature and then drips out of the apparatus, the number of seconds it takes to drip out is the viscosity, for example, 100 weight oil takes 100 seconds to drip out. SAE cuts the number in half. There are other ways to measure the viscosity in this day and age, I'm sure.

The W stands for "Winter" and "multi-grade" is kind of a lie. If you consider that a single grade oil (say, SAE50) becomes less viscous as temperature goes up then you could draw a curve showing this relationship. You could draw the same curve for a lighter weight single grade oil (say, SAE20). In very simple terms, multi-grade oil is simply an oil where the curve begins at the high point of the lighter weight oil, and ends at the low point of the heavier weight oil.

so 20W50 has the viscosity characteristics of SAE 20 weight oil (Seyboth 40) at colder temperatures, and SAE50 weight oil (Seyboth 100) at higher temperatures.

Clear as crude? ;)
Not really. In a 20W50 they use a base stock of 20 and add viscosity index improvers(VII) to get close to a 50 when warm. As the mineral oil is used the VII's break down and eventually you have a straight 20 weight oil.

More that anyone ever wanted to know - http://www.bobistheoilguy.com/motor-oil-101/
 

trafficinsight

Well-Known Member
Yes, that's how they do it, my point was that the oil doesn't magically get thicker as the temp goes up, viscosity still goes down but the slope is less, thanks to the additives.


Sent from 1865 by telegraph....
 

mshunter

Well-Known Member
Can someone explain how single-grade oil viscosity is measured compared to multi-grade? I understand that 15w50, for example, is 15 weight at cold temperature, but thins out like a 50 weight would as it heats up to 100C. But I'm not clear on how the single-weight scale works. For example, a 100w couldn't possibly mean 100 weight, could it?
NOTICE TO ALL READERS: This computer does not contain spell check, read at your own risk. It may make you dumber.


Yes, 100wt oil is really 100wt oil. It's VERY thick when cold, but thins out quite a bit when it's warmed up. It desgined to be 100wt at a certian temprature range(roughly 210*f). Next time you are out flying, look at what the temp of the oil is in cruise. It will be between 180 and 220 as long as the engine is in good health. Multi weight oils main advantage is that when they are cold, they stay "thinner." So on cold starts, the oil flows more easily to where it needs to be, you get reduced wear, and you are less likely to pop an oil filter. I learned the hard way that running to thick an oil is bad for oil filters when I owned my Baja Bug. I had added an oil filter, but it was before the bypass, so it got striaght oil pressure from the pump, and I burst an oil filter on a "cold" (I live in SoCal, it doesn't really get cold here) morning.


Thicker (higher weight) oil is usually worse for an engine than running a thinner oil. With modern engines, the tolerances are tight enough that 0w30 or 10w30 will suffice just fine. The thicker oils also cost more horse power to pump, so fuel efficency goes down. An example would be my brothers race car. Switching from 20w50 to 0w30 on the chassis dyno netted almost another 100hp on a recent dyno session. And as a R.O.T. (rule of thumb), you only need 10psi of oil pressure for every 1000RPM. Anything more and you are wasting horsepower.

This guy knows: http://www.bobistheoilguy.com/

I know a lot of people have heard this one before, but if you run a good oil, you can go signifigantly longer between oil changes than what is recomended. I have a Ford Ranger with a 3.0 v-6 (Vulcan engine as Ford calls it if I remember right). It has almost 200k on it now. I bought it with about 40k on it. I have changed the oil about every 25-30k miles since I have owned it. But I change the filter about every seven. I saw some oil studies when I was an automechanic that really changed my mind on how to take care of my vehicles. You can get 25k easily out of a synthetic oil if you keep a good filter on it. I buy a case, keep the oil level topped off, change the filter every 7k or so, and when I run out of a case of oil, I do an oil change. I usually only have to add a quart and a half when I change the filter to top it back up.


Basically, follow the recomendations for what kind of oil the manufacturer says to use. If you find yourself having to run a thicker oil than what is recomended, then there is likely something wrong with your engine, and plan on a replacement in the near future. It's like fixing an amputation with a bandaid. You are still going to bleed out, it is just going to take longer depending on the size of the bandage.
 
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