I had squeaky seat syndrome in my '85 190E. It was the plastic covers around the hinges rubbing on the leather seat. After some failed attempts with leather conditioner, mink oil, and carnuba wax, I tried some super thick silicone that worked. The stuff was meant for detailing rubber, tires, plastic, etc. I don't know the brand because the silicone lubricated the label off the bottle years ago, and I haven't seen it in stores since. It was pure clear silicone and thick as honey.
I'm not sure if silicone grease would hurt leather or not, but that is what I would've tried next. My new car does the exact same thing, I think I'll go fix it now while I'm thinking about it.