Yes, there is a seal -- really a plunger fitting is a sleeve with oil pumped into a hole. When the plunger is pushed down past the hole, it seals off the oil in the lifter, and there is a small check valve (on top, I think) that allows low pressure oil flow, but not high pressure -- the higher flow causes it to close.
If the seat of the small valve or the bore or the plunger is worn, oil leaks out and doesn't refill fast enough.
Hard sludge from infrequent oil changes will do this, and believe it or not, there are lease cars that have NEVER had the oil changed during the lease -- just gas, nothing else. Why bother, if it isn't blowing oil smoke on return, who cares?
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!