[Walt Fricke]

If the external oil thermostat is open (oil is hot), but there is an obstruction in one or both of the front cooler likes (as in one is crushed flat), then that is where the pressure relief inside the thermostat housing comes into play - it opens, and the oil heads straight for the tank.

I don't think many people run oil in one side of this type of cooler and out the other side. For most of our cars this complicates the plumbing. Porsche does not do that with the 964 oil coolers. Didn't do it with the RSR cooler, did it? Light airplane coolers are built out and back also - I used two of them in my GT4 car at one point. Fittings all just on one side.

The resistance in the system is just not something anyone has worried about beyond using large enough lines and fittings. Think of the extra trouble you are contemplating here - somehow opening up the part of the casting on the far side so you only need one exit fitting. The near side is probably easy - you can drill using the fitting hole which is vertical. The across and back design works fine in our cars.

If there is any benefit to in one side and out the other, I'd expect it to be that all the oil is at basically the same temperature - getting lower as it passes across, but still all as hot as it can be. More like a parallel setup. The out and back system is more like a series system, with the cooling efficiency getting progressively less as it turns around and starts back.