Understanding weight ratings

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May 25, 2003
Rancho Cucamonga, CA
Based on what I've read, the first number (the "W", or "winter" number) is the oil's weight at cold temps, while the last number is the weight at high temps. Based on that, should someone who lives in perpetually cold weather ignore the last number, and should someone who lives in a hot climate ignore the first? Let's say I live in a warm/hot climate (I do!). Would there be any difference in using, say, a 5W-30 as opposed to a 10W-30?
The last number describes the actual pumping viscocity range of the oil when its at 100°C. If it falls in a 30 range its called a XW-30. The first number is determined by the cold temperature pumpability I believe. You'll find that sometimes oils which appear to be labeled thinner, like a 0W-30, actually have a higher 40°C Viscocity than a 10W-30... The reason they are called 0W-30 is not because of this low temp viscocity, but because they maintain pumpability to a very cold temperature. Anyone want to back me up? [crushedcar]
Yup,thats as i understand it.0w rating of oils is a bit confusing, you think its a thinner oil at start up temps than a 5 or 10w but it is'nt necessarily so.Maybe it would be better if the cold end was measured at say 0 degrees c and not 40.
Originally posted by timzak: Based on that, should someone who lives in perpetually cold weather ignore the last number
No. Even in cold climates, the engine will eventually become hot (~200F) and then the second number is important.
Originally posted by timzak: should someone who lives in a hot climate ignore the first?
Yes. Someone living in hot Florida or California could use a 10W oil without any problem, because it never gets to -10F.
Someone once told me that if I switched from 10W-30 to 5W-30, my fuel economy would increase (due to less friction). From what I've been reading in this thread, that would not be true, since the extra "thinness" would only benefit when cold (on start up). Is that logical reasoning?
Timzak, The temperature shown on the gauge on your dashboard is not the engine temperature, whatever that is. It is the temperature of the coolant in the cylinder head. The oil temperature will be significantly cooler during cool weather easy driving, and the oil will be significantly hotter during hard, hot summer driving. Yes, 5W-30 will give lower fuel consumption when the oil remains cool. Dominic, The "_W" viscosity rating is indeed determined by the viscosity when cold. The unit of viscosity for this test is the centiPoise, and for example, a 5W- oil must be no thicker than 6600 cP @ -30°C. 10W must be no thicker than 7000 cP @ -25°C. http://www.chevron.com/prodserv/NewOronite/library/li_viscosity_motoroil.htm Cold borderline pumpability is a different temperature. Ballbearing, 0W oils are indeed thinner when cold...the viscosity must be no more than 6200 cP @ -35°C. Ken
Correct. You will get slightly better fuel economy with 5-30 than 10-30 because the oil will be thinner when the engine is cold. As far as climate...I guess if you live in FL, if you use a 15-50 vs. a 5-50 you will run into the above scenerio...otherwise, you can ignore the first number and use either. In cold climates...as was stated, the engine does still get cold, so you need the higher number as well as the lower one to ensure proper viscosity at extreme cold.
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