You didn't say how old the plugs were or the mileage on them? Also are the wires original and how many miles on them? One thing I did notice and I have seen others running Autolite plugs in the MB's;but the owners manual does state that Bosch supers are the prefered choice.
Try to always buy your gas from a station that has a huge turnover bussiness. The gas is fresher and you are less likely to get bad fuel. Top names such as Shell Exxon Chevron etc are good choices. The times I have gotten bad fuel were cut rate bargain places. A couple of cent per gallon is not going to break you anyway.
1989 420SEL 105575 miles
1990 Chevrolet E-Cab C-1500
1989 Olds Toranado Trofeo'