The M110's are great oil burners as they age. It could be seals, guides, or rings or all of the above. You need to do a compression and leak-down test to tell for sure.
You can try the seals alone, but you need a good valve spring compressor that will work with the OH cams, and compressed air blown in through the spark plug hole to hold the valves up.
The Haynes manual "Mercedes Benz 280 123 Series" has a good section on the engine.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe