The 40 is not necessary but many commercial lawn services do this in the hot south … was using Delo 10w30, but with just two quarts left, decided that's on hold for my boat.
Had a partial jug of the Delvac leftover … so changed, ran, drained, filled again to not mix oils …
(left filter on).
As for a seat of the pants dino test … mowed yesterday and it ran great …