A finned tube bundle heat exchanger utilized for heat transfer between a pair of countercurrent circulating liquids, such as for an oil cooler, wherein a bundle of tubes for the flow of one liquid therethrough pass through spaced openings in a plurality of fins or plates which are in heat transfer contact therewith. The fins are provided with a plurality of slits extending over a substantial portion of the fin surface to provide fluid flow directing openings and/or act to interrupt the fluid boundary layer on the surface of each fin and enhance the heat transfer characteristics of the finned tube bundle.