We recently had a rather alarming experience in Europe and would like to pass along a warning to other innocents abroad. Upon arrival at De Gaulle Airport in Paris we went to the Avis counter to pick up our rental car. Because we would be dropping off the car at Zurich, the clerk said she had a Swiss car, and that even though it was a higher grade (a Mercedes 190) than we requested, we could have it at the same cost so it could be returned to Switzerland.
Three days later, in rural regions of the Loire Valley, our fuel was low but a gas station attendant said she could not fit the hose nozzle into the car tank. With a Gallic shrug she told us that the car required unleaded gas but that it was not sold in France.
We were tempted to use a funnel to fill the tank with leaded fuel anyway, but elected not to risk ruining the engine of the expensive car.
Our hotel clerk called Avis in Paris and was told that the nearest city to change cars was Orleans. The drive to Orleans with the gauge on "empty" was a tension-filled experience that we do not recommend. Being carefree roamers, my wife and I could have been stranded on some farmer's back road.