Dirt on back lens

Hi,

I have just bought a used lens. I was cleaning the exterior making it free from a little dirt, just making it look a little bit nicer, when I unfortunately slipped and made a small fingerprint on the back lens element that is poisoned closest to the sensor (of film). I've heard that this lens element I covered with oil and you shouldn't touch it but now the damage is done. Should I go to a professional to have it cleaned? Can I clean it myself? Should I even bother? Do someone know a website about this?

Happy for response.