I went down and started the car up with the hood opened, and once the oil got warm oil started seeping out of a little hump in the middle of the top tank, it looks like a solder repair that I'd never noticed because it was painted like the rest of the tank. From the comments above, I gather that the tank is aluminum. The fins also? I think the first repair I'll try is to ream the pinhole out a little, just down to bare clean metal, and then try some two-part epoxy putty. I've had amazingly good luck patching leaks with it. I'll let you know if it works. Thanks for all the comments. In preparation for maybe having to take the cooler off for repairs, I think I'll start it in on a regime of penetrating oil.