My guess would be the crankcase vent and/or vapors reversing when the engine is shut off. This is common in any vehicle but shouldn't be excessive. The dealer is full of it if he's claiming it has to do with octane. Pick up some throttle body cleaner and give it a wash.

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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