I saw that people use the tags "oil" and "coolant" to indicate an oil-coolant-leak question. Would something speak against replacing that with a new "oil-coolant-leak" tag?
I would advise against it for the simple reason the meta provided by the oil and coolant tags would be lost to this new tag. There is a lot of cross over between the two when it comes to there being leaks one way or the other. By throwing out a new tag now, that meta data would be lost.
To my way of thinking, when you add a new tag, the idea is to broaden the scope of the meta data. For instance, we have a tag for chevrolet and one for gmc trucks. If we were to use these two tags in the way you want to move forward, it would be like creating a chevrolet-gmc-truck tag, which wouldn't really get us anywhere or improve the meta data. It would just confuse things. The two are related, but putting them together really doesn't help anything.
Also, as @Chenmunka states in the comments, another tag seems excessive to me as well. Would a new tag provide any more meta accuracy than the way it is? I don't really think so.