If you want the greatest oil cooling to occur, it's very important to have maximum airflow over the oil cooler. A smaller oil cooler placed in maximum airflow will cool as much, or more, than a larger oil cooler with blocked air flow. My 4"x4", Baker Precision oil cooler is very well protected, and receives excellent air flow below the front fender, and over the front tire.
After cutting a 4"x4" slot in the rear of the front fender, my oil temperatures were as low as they were when I tested the oil cooler with the front fender completely removed.
I experimented by drilling holes in the front fender before I cut out the slot; the holes did not allow much airflow, and greatly reduced the oil cooler's ability to radiate heat.
I recorded oil temperatures without a fender, and with a stock fender, before I cut any slots in my front fender. Using this method I recorded maximum oil cooling with a 4"x4" rectangle cut in the rear of the front fender.
The extra holes outside the 4"x4" rectangle were left over from my earlier experimentation. I don't think the extra holes help airflow, and they can be omitted.
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