The fuel in the regular tank of a motor vehicle is tax-free. In addition, motor fuel is tax-free up to the amount of 10 litres per motor vehicle if the fuel is brought into Finland in a portable container carried by a passenger car or a motorcycle and, if the fuel is of the same kind as the fuel used in that vehicle. Therefore, customs duties and tax are levied on fuel brought into the country in portable containers in commercial motor vehicles, such as trucks and vans.
Update, 16 June 2022: Due to the sanctions laid down by the EU against Russia, travellers are for the time being only allowed to bring in fuel from Russia in the regular tank of a motor vehicle. You are not allowed to bring in fuel in a portable container or a can carried in the vehicle.