I attended a winter driving school two weekends ago and overheard one of the older ladies talking to her daughter about her off-roading adventures - "in the rental, of course." My colleagues and I always drive our rentals normally, but I don't think the same can be said about most people. After that story (on top of having rented a couple of newer cars with worn-out suspensions,) I don't think I'd ever consider buying a rental car, even with low mileage and at a great deal.
With all of that said, I have asked, and every major rental company has replied that the oil is changed frequently. I've only had to add oil to a rental once, but the oil has always looked alright when I check.