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The first number is the viscosity when cold (w, stands for winter). The second number is the viscosity equivalent at temperature. No oil thickens as it heats. So 0w-50 has a 0 level viscosity when cool, and the viscosity when hot, is the equivalent of a single weight 50w when hot. And generally, a high resistance to thinning, will yield some more viscosity when cool as well. For example... Mobil1 0w40
Viscosity @ 40ºC, cSt (ASTM D445) 70.8
Viscosity @ 100ºC, cSt (ASTM D445) 12.9 Mobil1 5w50
Viscosity @ 40ºC, cSt (ASTM D445) 104.3
Viscosity @ 100ºC, cSt (ASTM D445) 17.1 Mobil1 15w50
Viscosity, @ 40ºC, cSt (ASTM D445) 125
Viscosity @ 100ºC, cSt (ASTM D445) 18 Mobil1 5w20
Viscosity, @ 40ºC, cSt (ASTM D445) 49.8
Viscosity @ 100ºC, cSt (ASTM D445) 8.9 Mobil1 10w40
Viscosity @ 40ºC, cSt (ASTM D445) 107
Viscosity @ 100ºC, cSt (ASTM D445) 16
So you see, 5w-50 and 15w-50 are pretty close at temp, but more different when cold, as their grades would suggest. I look at the second number as a resistance to thinning. Generally, the bigger the better, in my opinion.
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08:37:34 06/20/19
