I found this on VWVortex. This is what a guy posted.
quote:It is, of course quite a bit more complicated than that. I'll try to explain why I believe the low temp rating ultimately has an effect on the high temp protection of the oil. You are correct that the 0w gives the tested viscosity at a low temperature (w actually means Winter), and the 40 gives a high temp viscosity. However, the oil doesn't "turn into a 40 weight at high temps". It's one multiweight oil tested at 2 *specific* temperatures (like 0C and 100C I believe), and the two numbers say "At 0C, this oil has the same viscosity that a 0 weight oil would at this temp, and at 100C, this oil has the same viscosity that a 40 weight oil would at this temperature). All oils no matter what their weight is, get *less* viscous as they get hotter. ALL OF THEM. A straight 40 weight oil (would be 40w40.. meaning it behaves like a 40 weight oil at 0C, and like a 40 weight oil at 100C) is getting thinner and thinner as it heats up. That's just how oil works. It can't actually be *less* viscous at 0C than it is at 100C, but it can behave like a less viscous oil would.. The trick for making multi-viscosity oils is to create molecules with long wound up polymer chains, that uncoil and get longer as they heat up. When they're cold, the molecules are compact and thus less viscous (more runny). As they heat up, base oil gets less viscous as stated above, but the molecular chains stretch out causing the oil to become more viscous relative to how its shorter chain version of itself would test at high temps. Here's what bothers me. This whole stretching trick is neat, but it ultimately has a limit. The molecular chains can only stretch out so far. The 0w base viscosity should remain fairly constant as the temperature gets colder and colder because the chains can't get much smaller, but as it heats up, and gets longer, the tested viscosity must fall off at some high temp. Assuming the amount that the molecules can stretch is fixed (I'm sure it's gotten much better over the years, and is undoubtably better in the more expensive oils) then a 0w base oil will start to fall off at a lower high temperature than a 5w, or 10w or 15w. Since the industry only measures it at 0 and 100C (212F) and we know the *average* oil temp in a VR6 can reach 260+F, and where the oil is in contact with the pistons and cylinder walls, it's 400+F, how far do you think a 0w oil can stretch it's molecules to provide 40 weight protection at high temps? How high? A way to illustrate what I'm talking about is this. (let's try it as a picture instead of ascii) horizontal axis is oil weight vertical is temperature and each value in the matrix is the tested viscosity relative to the industry standard 0 weight oil. The 200C row is interpolated to show what I *believe* to be happening, and isn't based on any test, but simply what I know of the way oils work. 0C and 100C are known, but my concern is that at 200C, the multi viscosity oils will all show lower viscocities compared to the straight single grade oils and that a 0w40 is less viscous than 5w40 which is in turn less than 10w40 and 40 weight and 15w50.. etc. Hope that makes sense. If they made a 10w40 synthetic, I'd probably run that, but they don't, and 15w50 is just too thick, and always made my car run *hotter*, which put added strain on the cooling system because the VR6 uses a oil/coolant heat exchanger to cool the oil, rather than an air/oil radiator
originally posted by InProJettar » The 0 is the viscosity of the oil when it is cold, once it warms up, the viscosity becomes the 30, 40, or what have you. I worked at a lube place and they show you this video and make you take this goofy test with all this sh*t on it before you work there.