M1 0W-40 SN has a very good kind of inorganic friction modifier (trinuclear moly) and a good dose of quality organic friction modifier. Also don't forget that it uses an excellent, low-friction base oil. It used to be rated SL/Energy Conserving until the latter part of the spec got obsolete.
On top of that, many OEMs have been jumping into the xW-20 bandwagon, even if it could actually result in worse fuel economy because of increased metal-to-metal contact caused by thin oil. Chances are that, despite the OEM recommendation, the OP's engine likes thicker oil better to begin with, and there is no more fuel-efficient thicker oil than M1 0W-40 SN with its light xW-40 viscosity (13.5 cP @ 100 C) and excellent friction modifier (FM) package as well as excellent, low-friction base oil.
On top of that, many OEMs have been jumping into the xW-20 bandwagon, even if it could actually result in worse fuel economy because of increased metal-to-metal contact caused by thin oil. Chances are that, despite the OEM recommendation, the OP's engine likes thicker oil better to begin with, and there is no more fuel-efficient thicker oil than M1 0W-40 SN with its light xW-40 viscosity (13.5 cP @ 100 C) and excellent friction modifier (FM) package as well as excellent, low-friction base oil.