The lifters aren't too hard to change, if they are like the ones on the M103 engine (I think you have the M104, dual cam right?).
It's probably worn out, they are known for relatively early failure (in comparison to the rest of the engine). You can either live with it until it clacks all the time or change it. Probably different oil won't help.
Since you've been changing the oil frequently, I woudn't think sludge buildup would be a problem. However, if you have a large amount of visible varnish (golden brown to black coating) visible through the oil filler cap, it might be causing some of the trouble. Using and "engine cleaner" could certainly dislodge hard sludge and cause some havoc, but I don't see how that would damage the oil holding capacity of a lifter. I'd expect it to stay down rather than pump back up!
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!