# How to understand oil spec?



## jetta20Valex (May 26, 2007)

What is the number before the W and after the W stand for?
People say bigger number means thicker oil, the number before the W or after the W is telling the thickness? 
For example, 10W40 and 5W50, which one is thicker? What is the difference?


----------



## shipo (Jan 3, 2006)

*Re: How to understand oil spec? (jetta20Valex)*

The first number (the "W") number is the reference to how an oil flows when it's cold. Roughly speaking, an oil that is a 10W-_something_ will flow like a straight 10 weight oil when both are cold. The flip side of course is that straight 10 weight oils thin out to such a degree that they are waaaaay too thin to safely operate when the engine is up to temperature. That's where the second number comes into play, an oil with a higher second number will only thin out to the point where it will flow roughly like a straight weight oil of the same weight.
Long story short, the following is generally true:
10W-40 equals 10 weight oil when cold
10W-40 is waaaay thicker than 10 weight oil when hot
10W-40 is waaaay thinner than 40 weight oil when cold
10W-40 equals 40 weight oil when hot

_Quote, originally posted by *jetta20Valex* »_For example, 10W40 and 5W50, which one is thicker? What is the difference?

When cold, a 10W-40 is thicker than a 5W-50
When hot, a 10W-40 is thinner than a 5W-50
Clear as mud?


----------



## jetta20Valex (May 26, 2007)

It is very clear!!! Thanks.
Someone say a old engine should choose thicker oil, I would like to know the thickness in cold or thickness in hot is more important for old engine? So, I should choose 10W40 or 5W50 for old engine?


----------



## shipo (Jan 3, 2006)

*Re: (jetta20Valex)*

Any multi grade oil, even 0W-20, is too thick for full power applications when cold, hence the lower the first number and the higher the second number (within reason), the better for an old engine. Said another way, 5W-50 is far more preferable for your engine than 10W-40.


----------



## jetta20Valex (May 26, 2007)

*Re: (shipo)*

Thanks.
How about for newer engine? Is it better to use 5W40 rather than 5W50?
Someone say thinner oil will fit the small engine(1500-1600CC) better, is it correct?
What is your recommendation for high performance NA and Turbo engine? Let's say Honda K20A Vtec which will rpm up to 8500rpm and Subaru STI Turbo.


----------



## shipo (Jan 3, 2006)

*Re: (jetta20Valex)*


_Quote, originally posted by *jetta20Valex* »_How about for newer engine? Is it better to use 5W40 rather than 5W50?

For a new or newish engine, I would absolutely follow the manufacturer's specifications regarding oil. Current VW specs include 0W-30, 5W-30, 0W-40 and 5W-40. I use 0W-40.

_Quote, originally posted by *jetta20Valex* »_Someone say thinner oil will fit the small engine(1500-1600CC) better, is it correct?

The size of the engine has absolutely nothing to do with the grade of the oil. Per VW's recommendations, I used Castrol GTX 20W-50 in my 1.6 liter Scirocco.

_Quote, originally posted by *jetta20Valex* »_What is your recommendation for high performance NA and Turbo engine? Let's say Honda K20A Vtec which will rpm up to 8500rpm and Subaru STI Turbo.

Don't know much about the Subie, however for the K20A, I would follow Honda's recommendations and use a good quality synthetic 0W-30. My choice would be Castrol Syntec (made in Germany) 0W-30.


----------



## wilgomez (Jul 18, 2006)

I use Castol SLX 5w30 in my k20z1. i found that it does not consume oil as fast as mobil 5w30.


_Modified by wilgomez at 5:24 PM 8-10-2008_


----------



## franz131 (Apr 13, 2008)

*Re: (wilgomez)*

'W' numbers are determined by the temperature that an oil falls below a set pumpability level. The lower the temperature the lower the number.
The high temperature number is a range measured at 40c and 100c, the range part is what causes confusion because 1 xw30 may be much thicker or thinner than another.
Euro oil standards also include another measurement beyond the API temperature range above. Its called "High Temp, High Shear" and it's measured at 150c, well beyond the 40c/100c temperature measurement. HTHS measurements are the best way to differentiate the suitability of one oil vs another as it is a true measure of the working viscosity of the oil.



_Modified by franz131 at 6:25 PM 8/11/2008_


----------



## child_in_time (Aug 9, 2006)

SAE (Society of Automotive Engineers) classifies oil according to viscosity. Each viscosity band is specified at only one temperature. SAE 20,30,40 and 50 are specified at 100°C (212°F) whereas SAE 5W,10W and 20W are specified at -18°C (0°F).
Multigrade oils must conform to a viscosity specification at both temperatures. For example, san SAE 10W-40 oil must satisfy the 10W viscosity requirement at -18°C and the SAE 40 requirement at 100°C.
In more simple words, the first number indicates the viscosity of the oil at a cold temperature, while the second number indicates the viscosity at operating temperature.
Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.


----------



## saaber2 (Jul 15, 2008)

*Re: How to understand oil spec? (jetta20Valex)*


_Quote, originally posted by *jetta20Valex* »_10W40 and 5W50, which one is thicker? What is the difference?

Another thing to consider is the range between the low and high numbers. Many oils rely on Viscosity Index Improvers (VII) to gain a greater range. VII's can be susceptible to shear. So in general, you would find less VII's in a 10W40, where the numbers are closer together, than a 5W50, where the numbers are farther apart. But every oil has a different formulation so that is just a generalization and it depends on the specific oil brand and viscosity. 
Read about it here:
http://www.bobistheoilguy.com/oilshear.htm


----------



## shipo (Jan 3, 2006)

*Re: How to understand oil spec? (saaber2)*


_Quote, originally posted by *saaber2* »_
Another thing to consider is the range between the low and high numbers. Many oils rely on Viscosity Index Improvers (VII) to gain a greater range. VII's can be susceptible to shear. So in general, you would find less VII's in a 10W40, where the numbers are closer together, than a 5W50, where the numbers are farther apart. But every oil has a different formulation so that is just a generalization and it depends on the specific oil brand and viscosity.

As a general rule, synthetic oil doesn't need to rely on viscosity improvers, and as such it is far less inclined to "shear down". I stick with my recommendation of 5W-50.


----------

