I would not touch it if it is not causing any problems. Not sure I would even try as there is an easy chance of scratching the mirror, or a slip which could damage the shutter. if you really want it cleaned, then send it in to Nikon and have them clean it. I am somewhat surprised you have a build up of oils in the mirror chamber, as I have nikons, and another brand of cameras that are over 10 years old and not a drop in that area. If it is just dust, then using the Rocket Blower or another hand blower will clear it up, use air only on the mirror, as they scratch VERY EASILY.