Most of the cars on those roads will be rental cars - don't worry too much, but keep in mind you can mitigate some of the risk by not booking a small economy car. I agree with the other poster, do a quick run through a car wash before returning to remove any evidence, I've done this plenty of times in Hawai'i.

Obviously extra care is needed - watch for pot holes which can be deep, and other road debris, but many of these roads are well traveled even if unpaved, so you're unlikely to have any problems. If you're a auto club member in your home country, even better - in case something happens, you can have the auto club take care of the tow instead of calling the rental car company.