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Oil coolers are usually mounted in front of the Rad... above oil pan height almost always. I don't see how "drainback" would be an (added) problem. The rest of the engine is above oil pan height also.

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In my experience, they all drain back over time, check valves or not - including the factory installed units. It just amounts to a longer time before oil pressure comes up. Is that bad? Well, it aint good, and a poor trade off if you don't need to go there. Obviously, stock motors with lower compression and tighter clearances would suffer less than the other way around. My tow vehicle has one. Good application there, especially where I live - plus it doesn't sit for long periods of time between start ups. My race car ( with a filled block ) does not - and doesn't need one. If the OP KNOWS he is running outside (hotter) what his oil temp range should be, he needs one. Otherwise, he don't. IMO,in many cases they are installed unnecessarily. I was going to install one on my race car, but decided to install an oil temp gauge first, just to see where I was at. Good move on my part ( for a change ). I was well within the oil temp parameters for the oil I am using.