I had no problem with siezed glow plugs during a complete changeout in the 1987 300D. These glow plugs are the same as used in the earlier 1981-1985 iron head diesels, but in the aluminum head 603 diesel. No problem turning them - a bit of a problem fully removing them due to packed carbon. But gentle persuasion and twisting loosened up the carbon. Followed that with a glow plug reamer and the new plugs went in easily.
But of course I used anti-sieze on the new ones.
I agree with JCUYHN that its more a design/dimension issue than a materials issue, based on all the descriptions I've read. Afraid I'm scratching off the later MB diesels from any future plans.
So we got "rod benders", "head crackers" and "glow plug grippers" to avoid.
Ken300D