Motor vehicles purchased or leased as provided by section 307.41 of the Revised Code shall be for the use of the county commissioners or other county officials, their use to be subject to the regulation of the board of county commissioners. Vehicles shall be used by the officials, deputies, and employees in lieu of hiring vehicles unless the county vehicles are not available for such use. When vehicles are so purchased or leased, the board may purchase or lease supplies as are necessary. All vehicles shall be plainly and conspicuously lettered as the property of the county. No official or employee shall use or permit the use of any vehicle or any supplies for it, except in the transaction of public business or work of the county.