The valve adjustment interval is depending upon my faulty memory -- 15,000 is the interval for diesels, and I cannot remember if the gassers are different or not.
They don't wear much, it's not a huge problem. PITA to adjust, though you MUST use a special crow-foot wrench that is ground flat on the bottom, or you round off the adjuster, requiring that you remove the rocker arm to make adjustments. Hydraulic tappets are so much nicer....
The cam is oiled by a steel tube alongside, held onto the cam towers by plastic inserts. The inserts get hard and brittle, so ANY time you mess about with them, replace the plastic bits. The cam will go very fast with no oil .... !
Check the oiling by running the engine for a short time any time you have the valve covers off, making sure you see oil dripping onto the cam from the oil tubes. Don't rev it, you will spray oil everywhere!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!