quote:Because multigrade oils are a new technology, relatively speaking. Back in the olden days, there was just plain, straight-grade oil which got one number, say 30 weight. A 30 weight oil, of course, is thick when it's cold and thin when it's hot, but whatever temperature you pick, it's thicker than a 20 weight and thinner than a 40 weight. Now, somebody invents what we call 10W-30. When it's hot, it's as thick as our old 30 weight. But if we put it in the freezer, it doesn't thicken up as much as our 30 weight... relative to a straight 30 weight, it's thin when cold. That's why it gets a lower number, 10W-30. I think you're thinking of "30" as a direct measurement of viscosity. It's not; we measure that in cSt or other units. Just think of "30" as an old-technology reference oil that we compare our oil to, not a reading you'd get from an apparatus that measures viscosity. Edit: I didn't mean to suggest that turbochem doesn't understand this. My explanation was really meant for the original poster. [ July 02, 2003, 03:02 AM: Message edited by: mph ]
Originally posted by turbochem: I see what you're saying.....why is it when the oil is cold, and thicker than when warm, do they give it a lower number??
quote:Actually, all oils (even straight grades) that meet modern API requirements have an "additive package" comprised of detergents, dispersants, and extreme pressured compounds. Multi-vis oils have an additional additive, the VI improver. (However, some synthetics may not have this if the base oil blend itself has a sufficiently high VI to cover the desired vis spread.)
Originally posted by Problem Solver: Multi-vis oils, whether petro or synth, were designed to have much better pumpability in the winter, thus the lower number on the front-end (5w-30, the "w" stands for winter); the additive package is such that it makes dry start-ups less abrasive on your engine. On the flip-side to that, what they mean by "30" is that under heat and stress the oil will perform like a 30 weight not that it truly is a 30; this sounds a lot like the post up above by "MPH". Multi-vis oils require an additive package, some are more extensive than others, however, you should be looking for an oil with a high TBN and minimal additives; you want your engine protected by the base oil and not the additive package, afterall, the additive package is the first to boil off.
quote:On this chart, yes. BUT, that doesn't mean that every oil that has a vis of 9.4 cSt @ 100°C will have a vis of 80 cSt @ 40°C. That isn't what the chart is trying to show.
Originally posted by Ken4:quote:80cSt @ 40C = 9.4 cSt @ 100C = SAE 20 ???
Originally posted by G-Man II: I don't think you're reading the chart correctly.
quote:Exactly. You will never fully grok the meaning of "double-you" until you give up the notion that the left number of the multi-vis rating bears any relationship to the right number. The SAE should have used letters instead to represent the cold rating. It would have short-circuited much confusion. [ July 02, 2003, 11:34 AM: Message edited by: Jay ]
Originally posted by turbochem: I guess they could have used letters instead.....
quote:That's why when I was a kid, and someone tried to explain multi weught oils to me they said something like "Well, when it heats up, it gets thicker." Obviously that 30 being higher than 10 threw their reasoning a bit off....the oil is not thicker at high than at low temps.....
Exactly. You will never fully grok the meaning of "double-you" until you give up the notion that the left number of the multi-vis rating bears any relationship to the right number.