Well I feel like a stranger here, among your E34s :stickoutt... but to the point: my dad has a '90 E34 520i (yes it is the slowest thing on earth), a few weeks ago the engine started vibrating a bit, idling and low rpm acceleration felt rough. First I went to check the plugs and replaced them all, the strange thing was that from one cylinder the plug came out filled with some sort of glue-ish black thing, some sort of a mix between oil, carbon, gas and dirt. I just thought "well, that was it, one plug was bad"... I bought a new set, same as the manual said, but the roughness was still there, today I checked the plugs again and I got the one from that cylinder all dirty again. Is this an evidence of a serious engine problem? Have you seen something similar?

stuck open injector maybe?

:help May be, the car had the injectors cleaned recently so a bad done job is possible... it looks like when you had the mixture too (TOO!) rich in a carburated car, it's a slimey black thing, not just oil... seems worse.

Pictures of said plug would help. I wonder if you have a burnt valve, blown head gasket, misfiring cylinder, etc. I would start with a standard tuneup. (other than plugs) so rotor, cap, wires. (assuming your dad has the M20 engine) If it is the M50 version then the approach will be different. Coil packs etc.

I would next do a compression test - if the compression is OK - probably replace the injector.

Well I feel like a stranger here, among your E34s :stickoutt... but to the point: my dad has a '90 E34 520i (yes it is the slowest thing on earth), a few weeks ago the engine started vibrating a bit, idling and low rpm acceleration felt rough. First I went to check the plugs and replaced them all, the strange thing was that from one cylinder the plug came out filled with some sort of glue-ish black thing, some sort of a mix between oil, carbon, gas and dirt. I just thought "well, that was it, one plug was bad"... I bought a new set, same as the manual said, but the roughness was still there, today I checked the plugs again and I got the one from that cylinder all dirty again. Is this an evidence of a serious engine problem? Have you seen something similar?

What about the early 518i, that has about 16hp less than the 520i

Judging by the description of the plug it sounds either oil-fouled or aluminum fouled. Get us a pic of that plug pronto, if it's got aluminum all over it, you've got major detonation damage to the piston, and likely need an engine rebuild. If it's just oil, you're not too far off either way, because this means at best you've got a stuck ring, and at worst you've got a broken ring/scored bore.