The SAE has been considering high-temperature grade ratings of 15, 10, and 5, which would step down in HTHS to 2.3, 2.0, and 1.7, respectively. The KV100 range would remain the same as with the current 20-weight rating. I don't see the logic in auto OEM's spending hundred's of millions's of dollars in redesigning and proving out their engines to live on thinner oils to save less than 1% in fuel. Contacting surfaces on bearings and cams would have to get larger, which would tend to drive up the weight and size of engines. Size tolerances for bearings and journals in order to maintain acceptable clearances for the thinner oils would get tighter, which would drive up the cost of the components. Formulating a 10-weight oil that still has acceptable NOACK properties would probably lead to a more expensive oil.
In the end, I think we would end up with an infinitesimally more efficient engine that would cost as much more to buy as the fuel saved by using 10-weight oil over the life of the car.
Alternatively, I think the future may lie with higher VI, and not lower viscosity ratings. If we stayed with the minimum HTHS of 2.6 for 20-weight oils, and greatly increased the Viscosity Index, the benefit to fuel consumption by having much lower viscosity during warmup would be more than with a 10% decrease in viscosity at stabilized operating temperature.
Consider a garden-variety 0w20 readily available now: Mobil 1 0w20. It has HTHS of 2.6, KV100 of 8.5, and VI of 170. If the VI could be doubled to 340, and the KV100 set at 6.8 cSt, the resulting KV150 would be 3.8 (via Widman), which would yield in the ballpark of an HTHS of 2.6 when converting to dynamic viscosity and allowing for some shear loss. This would yield an oil that performs like a present day 20-weight at high temperature conditions, but the comparative viscosities at low temperatures are very different:
Vis Index -20C 0C 20C 40C
VI 170 1593 339 107 44.8
VI 340 151 66 34 20.3
Here is where big fuel economy gains can be made by using oil that is 1/2 to 1/10 the viscosity of existing oils at typical startup temperatures. And there would not be any need to completely redesign engines to maybe eke out a 0.5% improvement in fuel economy.
In the end, I think we would end up with an infinitesimally more efficient engine that would cost as much more to buy as the fuel saved by using 10-weight oil over the life of the car.
Alternatively, I think the future may lie with higher VI, and not lower viscosity ratings. If we stayed with the minimum HTHS of 2.6 for 20-weight oils, and greatly increased the Viscosity Index, the benefit to fuel consumption by having much lower viscosity during warmup would be more than with a 10% decrease in viscosity at stabilized operating temperature.
Consider a garden-variety 0w20 readily available now: Mobil 1 0w20. It has HTHS of 2.6, KV100 of 8.5, and VI of 170. If the VI could be doubled to 340, and the KV100 set at 6.8 cSt, the resulting KV150 would be 3.8 (via Widman), which would yield in the ballpark of an HTHS of 2.6 when converting to dynamic viscosity and allowing for some shear loss. This would yield an oil that performs like a present day 20-weight at high temperature conditions, but the comparative viscosities at low temperatures are very different:
Vis Index -20C 0C 20C 40C
VI 170 1593 339 107 44.8
VI 340 151 66 34 20.3
Here is where big fuel economy gains can be made by using oil that is 1/2 to 1/10 the viscosity of existing oils at typical startup temperatures. And there would not be any need to completely redesign engines to maybe eke out a 0.5% improvement in fuel economy.
Last edited:
