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There seems to be a misunderstanding of multigrade oils
The viscosity of multi-weight motor oil is specified using two numbers. The first number is the viscosity when the oil is cold. This is followed by the letter W (which stands for winter, not weight), which is followed by the number that indicates the viscosity when the oil is at operating temperature. The higher the number the thicker the oil.
"SAE 30 is SAE 30 no matter what the "W" prefix number is: 0W, 5W or 10W. This viscosity in centistokes (cSt) @ 100 degrees C is with the minimum of 9.3 cSt and a maximum of 12.5 cSt. "SAE 40 is SAE 40 no matter what the "W" prefix number is: 5W, 10W, 15W or 20W. The viscosity @ 100 degrees C is within the minim of 12.5 cSt and a maximum of 16.3 cSt. "SAE 50 is SAE 50 no matter what the "W" prefix number is: 5W, 10W, 15W or 25W. The viscosity @ 100 degrees C is within the minimum of 16.3 cSt and a maximum of 21.9 cSt. Low-viscosity oils flow better than high-viscosity ones—the lighter-weight fluid is easier to pump and therefore circulates faster through the engine's various galleries. Low-viscosity oils also maintain a lower oil pressure, but the oil pump delivers a greater volume through the galleries than it would with thicker (higher-viscosity) oils. Heavier oils also tend to operate at higher temperatures because the oil pump has to work harder to force the lubricant through the system. Oil does not compress readily, so the added pressure increases the temperature. In the end, high-viscosity oils maintain a higher oil pressure, but the pump delivers a smaller volume of oil. HTH Keith & Shawn
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