I always use premium in my MB's, they seem to run better overall. Occasionally, I'll buy a tank of 89 octane and I can sense the difference under heavy acceleration. Buy the w126, 300SE, you will love it as a freeway cruiser. Biggest adjustment for me driving my 1991 300SE was the second gear start and 'slow lumbering' off the line. If I have a need for speed, I turn to my 94 E420. Let us know the outcome of your 300SE decision! A picture or two would be nice.