The reason for the inlet being at the bottom of the cooler is not for heat transfer (well not that I'm aware of), but rather for better filling of the cooler and the removal of trapped air with in the cooler. Once full of oil (and evacuated of air), and if there were no drain back when not running, it wouldn't matter which side of the cooler the oil entered. Having air in the cooling lines or cooler could cause airation an frothing of the oil, which has it's own set of problem, so best not to introduce new problems if you can help it.
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