Bad experience which cost us almost 3000€ in addition to the rental. At the office pick up, the agency employee told us to put diesel in the 4x4, insisting 3 times to put in "black fuel" (in Iceland fuels are indicated by 2 colors, the diesel labeled in Black and gasoline in Green) and especially not to put the Green one.
Fail, it was the opposite, so car substitution on day 2 of the trip.
The agency denies having told us this and refuses to make a commercial gesture despite the fact that this fault is shared (they told us with insistence the wrong information, and we didn't double check).
Lesson of the day : don't trust blindly and check yourself, because if you have an issue you will always be wrong.