I had the same symptoms in the 300SEL 4.5 156k miles. smoking and generally un-performing well compared to another 280SE 4.5. (this comparison car is just fast and tight)
I wrongly assumed valve guides.
This car was a short runner to the grocery store up and down the hills of La Jolla, not enough oil changes, never had sufficient warm up.

My solution,
disassemble entire motor,
do valve job, check everything.

Here is the problem & solution (i think)
The ring grooves were caked with solid carbonaceous crud between the inside diameter of the ring and the inside of the groove. I had to break a ring and scrape the grooves clean. Note that the groove tolerance (up/down) was very tight (not worn) but the ring could not "work" meaning the expanding combustion gas could not move the ring out to seal correctly.
Commonly known as stuck rings.
There was not enough cylinder ridge to bore and a hone cleaned it up fine(thank goodness). The crankshaft mains were .003" so the same bearings are back in. Pistons and full floating pins are perfect. The rod bearings and bolts are being replaced and a set of Deves rings installed. New cam gears and guides etc. all seals, all rubber replaced

Bottom line about $1200 parts (my labor free)
Good for another 30 years.