I agree with Terry (Midnight Rider). If you want top notch engineering and a priority on function, you bought the wrong brand. Harley manages to make huge profits selling "style" and "tradition", so they see no need to offer state of the art function. Besides, the average Harley customer rides less than a couple thousand miles a year, so the bikes don't have to be very good to last long enough to get them out of the short two year warranty. Oh, btw, they do install a so-so oil cooler on Touring models at the factory, but not the Dyna's or Softails.
If you aren't seeing oil temperatures higher than 250°F in the tank, you don't really need an oil cooler. The Twin Cam engine design calls for a normal operating oil temperature of 230°F at a standard ambient air temp (approximately 70°F). And you don't want an excessively low oil temperature, since the oil must get hot enough to drive off the water and fuel that collects in the oil due to the natural operation of the engine. If you decide to install a cooler, get one with a thermostat so you don't have to worry about overcooling in cool weather conditions.
We get a lot of discussions on the site about heat, and many folks fail to differentiate between the temperature of the actual parts, and the heat felt by the rider (comfort). An oil cooler can help extend the life of the oil and reduce the amount of heat induced viscosity loss, but it won't do a thing about how hot the rider or passenger gets. For that matter, those engine fans that blow on the heads also won't keep the rider and passenger cooler. They will help maintain a more consistent temp of the heads, which is a good thing and a sort of mechanical insurance policy. That's one reason why a good water cooled engine can be set up with tighter tolerances and can last much longer than an air cooled engine, "consistent operating temperatures".
Jerry