Oil cooler is one of the easier mods to do that actually improves the longevity of your ride with only a modest outlay of time and money. HD one is complete kit at a fairly low price...JAGG is the OEM who used to make the ones for HD and may be a little higher, but they offer more mounting and cooling options. Contrary to most comments, even in standstill traffic, the coolers with intercooling tubes have vanes inside contacting more surface area to the oil flow cooling the hot internal oil to the outer tube temperatures even if ambient is only 105 deg F.
http://www.hdtimeline.com/oil_related_tips/1781-jagg_oil_cooler_install.html
http://www.hdtimeline.com/oil_related_tips/1781-jagg_oil_cooler_install.html