The RMS (Rear Main Seal) is not only possible, it happens to most 850's after awhile, especially if maintenance on the pcv (positive crankcase ventilation) hoses was not done...this includes keeping them clean (including the oil trap) and/or replacing the hoses if they are clogged. I too agree that you need a new mechanic. This guy is draining you and doesn't seem to know your vehicle as much as he's letting on. New Rotors at 5000miles is absurd. Incorrect torquing (as mentioned earlier) and/or a bad caliper could be the cause of all of this. Until both are checked and done correctly, this guy will continue to cause you to have to buy new brake pads and rotors as long as you keep your car and keep going to him. The oil leak may or may not be the RMS. More common is a simple and inexpensive repair called a turbo oil return line gasket ($8 part and about an hour of labor). Get yourself to a new mechanic and tell him all that's gone on. I would hate for you to abandon a very good car due to someone else's ignorance.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.