Our 2010 Prius II with 157 k miles consumes about 20 ounces every 10 k miles. As the miles pile up, it is natural for some consumption to occur. I check under the hood every Saturday as part of a 10 minute routine to stay on top of it. As it slowly drifts to the 2/3's level, I add some . The key with any car is awareness.
Avoid high mileage anything if you're buying a used car. Reasonable miles on a car is OK. Plenty out there to chose from if you take your time and do the research. We just reached 90k with our '11 Prius and no problems yet. I'm one of those who check the oil after the oil/filter change is done at the dealership to make sure the level is where it should be and that the oil is clean. I've had experience with a busy dealership draining the oil and then sticking in a new filter while forgetting to add the new oil. Humans can make mistakes when overworked or taken off task to do something else and a lot of these service guys are going full out the entire day. I always check, just in case. Same goes while I'm refueling. Check the oil. It's a quick and easy way to keep track of the level and a way to detect any oil consumption problem before it becomes more serious and expensive.
242,000 on my 2010 and I regularly run top tier gas and chevron techron in the tank. No signs of excessive carbon build up here. (Costco is my new secret spot for techron, half the price of walmart/autozone) I never buy the cheap off brand gas. Mobil or BP only. Kwik Trip (midwest) is also top tier.
As I said before, everything is clearer in hindsight. But if you are 180 miles from home at 9:30 Saturday night trying to think things out it is a little different. Called three towing companies of which only one answered. Renting a car and tow dolly at that time of night would have been a challenge. Check out the numbers for renting a one way car with tow dolly. See how close you get to $650
