I just went through this; I've heard for years that you should mount an oil cooler with with both ports up or with one port higher than they other because of potential air entrapment.
In my build it worked better to mount it upside down so I asked the question on the Aero-Classics tech email. Here's the response I got back when I asked specifically about mounting the Aero-Classic P/N 8000215 upside down, i.e. level, but with both ports on they bottom.
Terry,
Your question is very common and equally as common as the myth that aviation oil coolers trap air.
Aviation oil coolers do not trap air because like there is an air fin on the outside of the oil cooler there in an oil fin on the inside.
The purpose of the fin structure is multiple but to include increasing surface area, breaking up flows for increased heat transfer and ensuring even pressure drop across both sides of the oil cooler.
With that, everything flows in the direction of least resistance. So, air will not get trapped because oil will push air rather than oil because it is less resistant to being pushed out of the oil cooler over oil.
I edited the above a couple times while trying not to write a book.
Hopefully it is at least a little bit clearer than mud!
Please advise if you have any questions.