Dunno about yours, but most Mercedes engines have oil pumps driven mechanically by the crank, so the dealer's theory is not making a lot of sense.
I mean, didn't anyone suggest an oil and filter change?
Here is some good advice on flushing:
Engine Flush Recommendations
I don't see what you have to lose by changing the oil and trying one or more flushes followed by oil changes. Moly is supposed to be a good product.
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