A heat exchanger for cooling a machine fluid of a vehicle, and associated method. The heat exchanger can include a fluid inlet tank, a fluid outlet tank, and a plurality of heat transfer tubes connecting the inlet tank to the outlet tank. Each tube can include first and second substantially flat sidewalls, a plurality of internal webs extending between the first and second sidewalls, and a plurality of first dimples formed on the first sidewall, each first dimple formed over one of the webs.