I would say it is fairly rare yes. If you are not interested in racing it, I'm sure you could get a pretty good car in return for it in the marketplace.
I have it and have seen it a few times in the UCD. It is only rare if you want it and it doesn't show up for you , The Bentley i never saw until i traded for one , since i have it now i have seen it twice in the UCD in the last few weeks (Bentley). Sods law !!
But a zero miles Pennzoil could be considered rare !! As most players would get theirs in the UCD.