Help in understading multi-weight oils

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Dec 8, 2002
West Coast
I understand how the higher number (e.g., 20, 30, 40, 50) is rated in multi-viscosity oils, but don’t understand how one determines the lower “winter” number (e.g., 0, 5, 10, 15). What ASTM (or other) standard do you look at to determine these lower weights. For example, I’ve read where some people here have said that Schaeffer’s 700 15W-40 could really rate as a 5W-40 (I hope I’m not misstating this). In any case, I’d like to know how the cold number works in the same way I understand the upper number. Thanks, Darren
The Schaeffer's 7000 15w-40 could be a 10w-40 if they wanted to market it as such. Check out this link for how the multi-vis ratings are determined: As an example, a 10w-30 must have a minimum kinematic viscosity of 4.1cSt @ 100*C to even be considered for the 10w designation. No problem there as 30-weight starts at 9.3cSt @ 100*C. Then it must have a pumping viscosity no higher than 60,000cP at -30*C and a cranking viscosity no higher than 7000cP at -25*C. Also there must be no yield stress during the cold temp tests. [ March 14, 2003, 08:27 PM: Message edited by: Jay ]
Darren, So-called multiviscosity motor oil gets tested for its viscosity at both a cold temperature and at a hot temperature. According to the charts on the links unDummy posted, if the oil has a viscosity not more than 7000 centiPoise @ -25°C, then its classified as a 10W oil. If the viscosity is between 9.3 and 12.49 centiStokes at 100°C, then its classified as a 30 wt. This oil would be labeled a 10W-30. Schaeffer's tech data for their #700 15W-40 shows the cold viscosity a bit lighter than that needed for 10W. If there was such a thing (which there isn't), it might be rated as an 8W-40. Why do they label it as 15W-40?...don't know...maybe marketing reasons. Would most truckers buy 8W-40 or good ol' 15W-40 oil for his engine? Ken [ March 14, 2003, 08:34 PM: Message edited by: Ken2 ]
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