I'm looking at an R50 (2006). I love the car, but the current owners have been using regular unleaded for the past 20,000 miles. I know that the knock sensor on the car is supposed to compensate for low octane in times that one accidentally puts the wrong grade of fuel in the engine, but what sort of damage would be caused by sustained use of low octane fuel? Should I run? Or is this unlikely to cause problems? The car has only 40,000 miles on it currently.

Incredibly (at least to me), the owners claim that the dealer told them they could use regular unleaded without any problems; but I suspect they must have misunderstood.

 

Long term regular use can cause valve /head damage