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.

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

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??