The valve stem is indeed the part extending through the head from the combustion chamber to the rocker arm. There is a rubber seal (Viton actually now, I think) that prevents oil from being sucked down the valve stem and valve guide (the tube the stem goes in). These get old and leak, causing blue smoke on startup and excess oil consumption. If you use a liter of oil in less than 2000k, you probably need to have them replaced.
On early 601 engines, the guides could wear too much as well -- you will have idle problems from the air going down the worn guides and very high oil use.
Oil changes at 3000 miles (convert to kilometers!) for mineral oil or 6000 miles for synthetic base oils (Mobil 1, etc). Oil filter each change. Change air filter every 30,000 miles (or less if you live in a dusty area, as I'm sure you do when it's not raining for six months....).
Clean the brake pad slots in the calipers every 15,000 miles. Change automatic transmission fluid and filter every 30,000, make sure you also drain the torque converter. Change differential lubricant every 30,000 miles.
By this milage and age, I would check the condition of the ball joints and steering linkage joints up front, and the condition of the suspenstion links and subframe mounts in the rear. Steering problems and sensitivity to crosswinds indicate worn or bad parts.
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!