There are thermal dispersant coatings that claim to be about 15% more effective than bare metal, but you need a shop to spray them as they contain nasty stuff you don't want to breathe.
Techlinecoatings.com
Look at that place. I have my oil cooler coated as well as many other things, but did not do my FMIC as I ran out of the TLTD coating. (It's on the undersides of my pistons, inside and out of my engine block, transmission and my cam sprockets are coated with this stuff.)
Very durable, and for commercial grade coatings, the prices are not too bad.
A thin coat of flat black paint is reported to do much the same thing. It's called a black body emmiter, and the theory is black paint absorbs heat, and since it's on the outside of the cooler, that heat is then transfered to the air flowing over the cooler and it improves the cooling effect.
The TLTD that I used however does this and more. It works inside and outside to allow better heat transfer. (Some claim it turns your oil pan into an oil cooler, so my oil pan is coated inside and out too...) LOL
Here's what it looks like when done. (The red is candyapple powdercoat, black parts are TLTD, silver is ceramichrome that is not polished and black hot parts are "turbo X" a 2000f ceramic coating that has been very effective at lowering underhood temps.)
This bolt on T4 hot side and elbow were coated with turbo x, and it changed the engine bay from a blast furnace to actually pretty cool. I never did, but you got the impression you could touch the housings even after a running the motor hard.
I also coated the manifold and heat shields to keep as much in there as possible. It makes a big difference in my opinion.
