Cutehumor, the conflict over motor oil with your wife is a great opportunity to practice the communication and compromise skills you will both need to make it work. By the way, does she hoard something? Ex. clothes, shoes, handbags, perfume, toiletries, kitchen gadgets,food, etc.. If she does, then the compromise is easy; don't touch my motor oil and I won't touch your shoes. It is better that it is all out in the open. If not you get into secret buying patterns.
When people tease me, I laugh and agree that I am eccentric. What do I care what they think. People then generally like that because they feel freer to be themselves. Your inlaws will respect you more when they feel your confidence, and probably won't try to manipulate you as much when they see you can laugh off their teasing. Hoarding motor oil is funny. Why not laugh and enjoy!!
What a great idea! I am going to start requesting motor oil for Christman. That is way better (for me) than some overpriced tie or gadget I do not want.