Thanks for the help. The brown disc on there now has the center torn out and the o-ring is certainly not sealing anything, so oil could be getting in. I have a new cap and rotor on the way from those super-efficient guys at PartsShop.com, so I'll replace cap, rotor and brown disc and see what happpens. I remember removing the screw that holds on the rotor mount, but I couldn't pull it off easily. Guess I'll use a mini puller and see if that works.
'88 300E 265K
'91 300TE 155K