Hi 4LFarm
I too went looking for an oil cooler for an 8T No.12 Motor Grader. I come to find out Oils of today are lightyears beyond that of yesteryear.I use a good multi-grade 10W-40 oil. If you aren't going to put the Ol' D6 back to hard labor an oil cooler isn't really necessary anymore. My D318 oil cooler has been removed and the lines were looped back into the filter housing. I have also heard of others just plugging the line ports.
Good luck