ASTM D7589
https://www.astm.org/Standards/D7589.htm
OK, this one costs money, maybe there is a free download to be had in the ether.
Basically fuel economy savings from resource saving engine oils vs a base-line oil disintegrate with age in use.
For example; xW20 resource conserving provides a 2.6% savings when fresh, but only 1.2% after 100 hours in the sequence test.
Other grades drop 1% over the same time.
At some point, there would be no fuel savings, and extending the drain further to save a few quarts of oil, will cost you gallons of fuel.
https://www.astm.org/Standards/D7589.htm
OK, this one costs money, maybe there is a free download to be had in the ether.
Basically fuel economy savings from resource saving engine oils vs a base-line oil disintegrate with age in use.
For example; xW20 resource conserving provides a 2.6% savings when fresh, but only 1.2% after 100 hours in the sequence test.
Other grades drop 1% over the same time.
At some point, there would be no fuel savings, and extending the drain further to save a few quarts of oil, will cost you gallons of fuel.