Thank you, Jay.
I'm convinced airflow is one the most important factors.
My oil cooler has 16 square inches of surface area, and all of it is perpendicular to unobstructed airflow under the fender. In addition, my oil cooler is mounted away from the engine block and exhaust header. I think all of these factors make a significant difference in an oil cooler's ability to dissipate heat.
2005 XR650L: Shorai Battery Relocation, Spud Oil Cooler, XR650R C/S Sprocket, Reinforced Subframe, Chain Slipper Roller, Performance Design Lowering Link, Baja Designs Headlight, FMF Hi-Flo Header, ManRacks SD Rack, ManRacks Front Fender Farkle, CST Surge I Front Tire, D952 Rear Tire, Tusk D-Flex Handguards, Uni Air Filter, No-Toil Evolution air filter oil