Jane,
Your problem is most likley valve guides that are worn and leaking oil. Correct replacement cost would be about $2600.00 That would get your cylander heads rebuilt and a new timing chain. However I have had valve guide seals replaced on a 420SEL with successful results before. Either way i would have a qualified MB technician look at your engine first.
Good Luck!
Adam

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Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320