yes, warm oil will run out better and will take more dirt along. I drove about 25 miles on the Interstate to warm it up.
Your manual should give you the volume required (it was 1.4 quart for a W126) but it should be right at the edge of the filler hole. Once it starts running out, its full.
The intervals for changing are rather long (like 60,000 miles?). My M&R manual doesn't even mention it.
The plugs need to be torqued. Again on a W126 it was 37 ftlbs or 50 NM.
Somebody with a car like yours might know exactly.
There is an alu-ring under the filler plug that should be replaced. Mb will charge you prob. less than a dollar.
Hopefully your plugs are not as tight as mine were.