I'm not sure if your 300SEL has the same type of engine & engine controls as my 300TE, but I'll tell you what I've done to my car to restore the fuel economy. I found a couple of vacuum leaks at both ends of the hose leading from the idle-air control valve, replaced that hose and solved low end driveability and idle flakiness. Replaced the OVP relay (not cheap, but very effective). Fuel economy improved significantly upon replacement. Replaced the spark plugs, plug wires, fuel filter. Oxygen sensor is next (this week hopefully).

I hope some of this info helps you. The OVP relay replacement was an expensive, yet instantly effective solution. My TE has 202,000 miles and runs perfect now.