Usually I mount oil coolers vertically, so oil comes down from above, crosses through the cooler and up and out the other end. This way the cooler is always full of oil, and I then fill the oil pan to the proper level.
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A friend wants to mount a cooler horizontally/flat. My thoughts are that oil will be pumped from the engine through the cooler, and oil will be in the lines and the cooler; under engine operation, meaning the oil pan will be "short" by that amount under operation, unless overfilled to start with..........
So the oil pan will always be "overfilled" with engine off, and maybe at idle;
or "short of oil" with engine running.

Your thoughts on this, please. My friend deserves a second opinion.
Thanks.-Bob