What can I do when something was replaced instead of what I requested

Hello, It all depends on what was leaking. If the leak was on the oil cooler, or the oil cooler was damaged, then replacement is needed. I would really have to see the vehicle and observe diagnostic notes to know if the repair is warranted. The other side of the coin is that they should have explained everything in writing, asked for your signature to approve the work, and warrantied their work in some way. There is no reason you should be conducting the same repair twice. Always ask to see the old parts next to the new ones, this ensures that yours if off, and the new one is there. Something I have seen done more than a couple times, people like to mark their faulty parts so they know if that part is still on the vehicle when they get it back... maybe not a bad idea.