There are no "silver bullet" fuel or oil additives that will improve mileage or performance, assuming your engine is performing nominally. Chevron Techron can clean off some injector and valve deposits that can cause driveability problems, and this may improve mileage if the car is running poorly, but gasoline additives won't do much for an engine that is running normally.

A slight idle roughness on M103 engines is normal and is due to the relatively lean idle mixture (stoichiometric) that is required for emissions. Mercedes service literature states that the lambda equipped M103s, will not idle as smooth as non-lambda configurations available in some markets, which are set up with richer idle mixture.

As long as the engine has good throttle response and runs smoothly above idle, a fuel additive will probably not have any effect.

M103s will run satisfactorily on regular unleaded fuel in cool weather, but you need to watch out for detonation in hot weather city driving the the A/C on and coolant temps over 90C.

Duke