ATF is oil, P/S fluid is oil, Oil is also oil. A tranny cooler will work fun as an oil cool. I would install a thermostat to keep from overcooling and help the engine warm up in cold weather. Cold oil also hurts gas mileage and power.
Some trucks came with an oil cooler, most didn't. It used a sandwich fitting under the filter that moved the oil through a tube in a special lower radiator hose, using the engine coolant to cool the oil. If you use an 820-S filter, you have the oil cooler. If you use an FL1-A, you don't.
I use the factory oil cooler, dunno how much it helps but I assume it helps a little. I will tell you its pretty tight to get the filter in there and I have to use a strap wrench to loosen it (no room for any other type).