The terms here might be confusing. The thing that covers the hub of a wheel is what we old people call a hub cap. The plastic dish that covers the entire wheel is a wheel cover. The incomprehensively expensive little insert designed for aluminum wheels is often called a center cap, but functions as a cap for the hub. At least, this is how I see the terms used most often. I see the wisdom in spraying something protective onto the hub no matter what cap or cover is used, but I wouldn't want any of that to migrate to a surface of the wheel that would interact with the wheel bolts. If you aren't supposed to get anti-seize on those surfaces, oil or silicone or anything else that might lubricate seems like a potential problem.
Agree, I mean, it's the cap over the hub, the hub cap, lol. Still, the term has evolved, has generally come to mean the full cover. Can't fight the tide of opinion I guess. English is funny, take how we say the letter "W", "double-yew" I suppose. Or describing the word "continuum", which has a "double U". To someone learning English, what a conundrum, just a subtle change to the inflection.
