Ill start off by stating that im no engineer, not by any means. I am however a very mechanically inclined technician that works on a lot of mobile equipment that often deals with hydraulic systems retaining excessive heat.
From my experience with a typical fluid to air heat exchanger, a parallel flow of fluid through the exchanger is the most common method employed. I personally believe this has more to do with cost effectiveness, vs heat shedding potential.
I like John's analogy of pushing water through a hose. While a longer hose may provide more resistance to the flow of the water, the longer continuous length will provide better overall cooling efficiency. I think the increased resistance to flow might not be a bad thing either. It takes time for fluid to transfer heat to any material. Slowing fluid flow down is an old hot rodder trick to keep flathead Fords from overheating. You can move fluid too fast to effectively cool your system down, simply by not allowing time for heat transfer.
Case and point, my hot tub. Same heater, the tub will heat up faster on low pump setting than high because the fluid moving past it is absorbing more of the energy put out by the heater.
Heating and cooling go hand in hand. If you can transfer the heat to the fluid effectively, you can more effectively transfer the heat back out on the other side.