I never noticed it before. I was putting on my 18-55mm lens and noticed what looked like hand lotion on it. Then I noticed that there were three identical strips all the way around the mount. The only thing I can think of is this is some sort of lube. Does anyone know what it is? If it is lube, it's not doing a very good job since it doesn't seemed to be in a contact point between things that need to get lubed.