Yes, you may be the proud owner of a nice rental car (which, of course, you paid for with your credit card) but when it comes to getting gas, don't go to Finnish gas stations like Shell, Teboil or ABC. ďForeign Visas and Mastercards donít work in Finnish service stations. In some stations you canít even pay with your card at the cashier," states Financial Newspaper Taloussanomat.
In the Helsinki city area (map here), you do have the option of using your card - but only at Neste service stations. It has been said that Finnish gas stations are working on the situation and hope to support foreign credit cards soon. Until then, make sure to carry cash for gas or stop at an ATM.
On a lighter note about driving in Finland: Did you know that if you drive along Highway 1 and want to use Finnish roadside toilets, you have to text-message "OPEN" to the Finnish Road Administration? Seriously!