If it's just dust, a blast with a rocket blower should shift it - hold the body throat down while you squeeze the bulb to blow the dust out. May be worth doing it over a mirror so you can see what you're doing. If the spots are oil that has jumped from the mirror mechanism, you'll probably need to do a wet clean, using one of the many proprietary systems available. That should get rid of the spots- if it doesn't, it may spread them out a bit and let something like a lens pen sensorklear remove the residue.