Hopefully someone else can speak to the thick oil on your clutch.

I don't know for certain if this is true for an 87 Diesel, but on either of my 92 V8 cars the A/C will cut out if the serpentine belt slips enough. In my case, it was caused by bad belt tensioners. The symptom is that the compressor will cut out after a hard accelleration and will either stay off for a few minutes, or will stay off until I cycle the ignition key.

I would usually hear a brief shreik from the belt as I let off the pedal from a hard accelleration and then begin to notice that the A/C was blowing warm air...

Just a thought...

__________________
02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)