Is your SIL a slob in other ways besides car maintenance?
If so, I see the future... and you are paying a lot of bills to keep your daughter well cared for, and not just for car maintenance.
If not, then there are 2 possibilities:
(1) He truly does not understand car maintenance, even with good examples from you and his company truck. He thinks that you are extreme. Go easy on him - this may take years, and he probably will not change. Don't lose him as a friend over this.
(2) Maybe he is a good guy with good sense, but he is just messing with your mind and getting some free oil in the process! If he is a Yankee then that would explain a lot.
Seriously, don't let this car maintenance stuff cause "family issues". Your daughter will chose him over you, your wife will be upset with you, and you will be sitting there with your feelings hurt, wondering what happened because you were just trying to help!
You need to consider that they have their own life now, and they need to be responsible for themselves. For your family's sake, you don't want to build walls of resentment over little stuff, and for your sake you don't want to pay for everything that's not right in your eyes.
Pick your battles carefully, and strive for a win-win situation.
SWS