Originally Posted By: bepperb
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You make a good example of M1 5w30 being thicker than 10w-30 at boiling. I'd love to hear your theory of why this is...
It's very simple actually, no theory necessary, see the charts, below. First off, the SAE ratings are NOT absolute points on the viscosity/thickness scale. Any oil that when hot is between about 9 cSt and 12 cSt counts as a "30 wt" oil. No mystery there.
Second, I think you're making a commonly seen error -- mistakenly believing that hot viscosity is related to the cold "w" rating -- they're really not. The product that's an old favorite here, GC (German Castrol), comes only in 0w-30, and is in fact thicker than most all other 30s, irrespective of grade. It gets the 0w rating not for "thinness", but rather for its exceptional ability to remain liquid and pumpable at temps that would leave a lower-quality 5w-30 a solid lump of goo.
Finally, given the wide range of quality additives available today, and of course, high quality base oils, it's child's play for an oil formulator to make a 10w-30 that's much thinner when hot than another 5w-30. Did you overlook the Mobil-1 example I gave earlier? So, if you won't take my word for it (and that's OK), here's some hard evidence -- annotated screen captures taken from the Mobil website today. As you can plainly see the Mobil-1 5w-30 is significantly thicker than the Mobil-1 10w-30.
Any questions???