Like the title says..

We had a very in depth chat at work about oil viscosity and the SAE numbers on enigne oil;
eg:
15W 40 verses a
0w 40.

It started with a rep who said that the first number is the cold weight of the oil, and the 2nd is the hot (running operation temp) weight of the oil..
So how can an oil be heated up and gain viscosity weight??

Then someone said that the second number is a 'code' for the SAE test prodcedure to classify the oil..
somebody else thought is was the molecule size of the oil..
someone else thought it was a pressure ratio index..

So, whats what?
any help would be great.

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There is a lot to it and if you have a look at the first handful of pages in the oil thread in general maintenance it covers most of it. In lamans terms which i used to explain to customers that didn't know much about oils, the first number is the viscosity of the oil when cold and the second number represents the viscosity of the oil at operating temperature. An oil thins as it gets hotter, obviously the thinner it gets the less it protects to a certain degree. Therefore you want an oil that is quick to warm up, represented by a low first number, and then an oil which won't thin out too much as it gets hotter, represented by the second number.
So comparing a 5W-30 to a 20W50:
The 5W-30 will be a lot quicker to warm up than the 20W50.
The 20W50 will take a lot more heat to thin out than the 5W30.
Obviously different climates and different driving conditions require different oils. You wouldnt run a 20W50 on a car that runs in extremely cold climates as it would never warm up.
There is a lot more to it as far as SAE ratings go and what temperature certain oils are tested at depending on their group and other things.
Hope that helps you out some what.

Nothing to do with how fast the oil heats up. It's all about viscosity. A 20W oil will still get up to operating temperature (considered to be 100degC).

The first number indicates the cold viscosity of the oil. The "W" indicates that the viscosity was actually measured at the cold temperature (around 0degC). This number only really means anything in relation to starting the engine. The lower the number, the faster oil can be pumped through a cold engine, and therefore the less dry running, and thus less engine wear.

The second number indicates the viscosity of the oil at 100degC. The higher the number, the higher the viscosity, and the harder it is to pump the oil around the oil galleries and through the bearing clearances. Modern high performance engines have small clearances, and thus require a low viscosity (30 or 40) oil. Older engines, like the old L-series Nissan engines, run larger clearances, and can tolerate a higher viscosity (50) oil.

Have a read: http://www.upmpg.com/tech_articles/motoroi...sity/index.html

I never said the numbers represent directly how fast the oil heats up. I said they represent the viscosity. A factor of different viscosity's is the relationship they have with heat and heat transfer coefficients. An engine wears the most when it isn't at operating temperaute. Coolant and oil has the main influences with how long the car takes to get to operating temperature (along with other things of course). A high viscosity oil will take longer to get to the same temperature as low viscosity oil assuming all other variables are the same due to this. Run a high viscosity oil in a sub-zero temperature and see how long it takes to thin out.
You saying that a 20W50 and a 0W30 will both take the same time to reach 80 degrees?

another thing to point out for the sake of this argument is that the oils with the 2 numbers are called multi vis (viscosity) oils, hence they have 2 numbers. old single weight/vis oils only ever have 1 number

ok then so whats the go with say penrite SIN10, it's a 10w70 oil as opposed to a 10w40?

Just read the Wikipedia articles on motor oil and viscosity. They will explain it for you, although not very well because it can be a confusing topic. Or you can read my explanation of the numbers here (specifically, post #1173):

http://www.skylinesaustralia.com/forums/Go...&start=1170

As for SIN10...being a 10w70...compared to a 10w40...will have similar start up viscosities, however when the engine gets up to operating temperature (100 degrees celsius) the 10w70 will stay thicker than the 10w40 because it's additive package gives it higher resistance to the oil thinning out as a result of temperature increase.

http://www.upmpg.com/tech_articles/motoroi...sity/index.html

Thats a good read,
That covers most of my questions..

so, what are all the lettters??