Used to own a car rental business at M/cr Airport and used to pull the fuel and cdw on the Americans mostly.It is nothing more than a money spinner to cover the low cost of the actual rental.The cdw is a legal con,so to speak.The car is fully covered by the cars insurance,the rental agency just frighten you to take out the extra cover.Unfortunatly there isn't a lot you can do except refuse it,you can in some circumstances put it on your own insurance but to be honest it really isn't worth the extra cost to yourself or the utter grief involved,check your credit card will cover you but be prepared to suffer the Spanish inquesition at the desk.Regarding the paying for a full tank,always go full to full or change rental agency,nobody brings a car back totally empty.