A few years ago i wanted oil cooling too, but really didnt want to go changing everything and spending loads of money.
So what did i do?
I found a rather heavy duty looking heat sink, not literally heavy, but could do a good job of sapping some heat away. It was about 5or6" square, with 2to3" tall fins standing off it, and i set about bolting it to the bottom of my sump, using computer heatsink compound that sets like cement to make the perfect join. The heatsink needed a little work on the flat face to make it fit my sump nice and snug, and the sump had a captive nut welded in place to take the underhanging heatsink. When it was bolted on some fuel tank sealant was used on the captive nut to stop it leaking. Jobs a good'n! It noticably took longer to warm up, and the fans almost never cut in. Its important i think, to have the veins of the heatsink lengthways with the car to accept the high speed air underneath and make a good heat exchange. Just... God help the cat if ever you run one over, it'll come out the other side like mince meat.