RedOak, I too am a guy that like to do oil changes and also likes to help people. You'll have to learn to let this one go. I know it's hard to watch family squander money this way, but for the sake of peace in the family and your sanity, you'll have to back off.
Firmly tell your FIL that his car needs regular oil changes and that it needs to be topped off between oil changes, or else the engine will go south. Also tell him that you are not willing to go out of your way to check the oil or change it off his property bacause it's nuts to do so. Tell him in a way that he knows there will be changes in the way you help him. Then keep quiet, or else you'll turn into a nag. If anything happens in the future, you can remind him of what you said and he can then only blame himself.
Thankfully, my family trusts me in all areas of car maintenance and respects my skills working with cars. For close friends I offer my help only when asked, and even then I give them disclaimers about things that can go wrong, such as snapping rusty bolts, and that they're responsible for any such consequential repairs.
Since this is an interpersonal problem, and not a technical one, I'm very curious where your wife stands on this issue.