Wow, this thread became a bit bumpy, I am surprised by the topic, that it got a little heated (lol)

The fluid slows down at the entry and through the cooler because of line size, the 5/8" tube takes twice the amount of fluid volume for any given distance as the 5/16 line.

Think of it like squeezing a garden hose, with x amount of pressure water flows smooth, make the outlet smaller makes more pressure less volume.

when you squeeze more volume into a smaller line the pressure increases.

In my opinion the differences in temp from top/bottom -in/out are negligible.

If you are adding a cooler to a stock system, it is important to run the front line into the radiator first then to the auxiliary cooler and back into the rear line. If you are using only an auxiliary cooler, in and out doesn't matter as much as size and cooling fan capabilities.

my Street/Strip Coronet front line into the lower cooler inlet.