Jane> You should be able to get the oil "scent" out of
your washing machine by running several empty loads using
only vinegar, then run an empty load using only hydrogen peroxide,
and then run an empty load using only bleach.
One, all or several of these things should work. You can see which one works best for you by using all three, one at a time.
In the future, you can pre-treat his coveralls with mechanic's "Mojo", (the stuff they use to get the grime off their hands), by soaking them in a tub of water and rinsing them off before they go into the washing machine.
Each time he or you wash those oily cover-alls, you should then run an empty load or two with bleach, vinegar or hydrogen peroxide, whichever worked best for you.
Give it try today, and let me know if it did the trick.