I have put in excess of 265K of the 410K miles on my 1995 E300. the mileage fell to about 28mpg @ 375K and i started looking into why. After replacing the injectors, replacing the timing chain, replacing all filters as well as the small plastic lines around the pump and filter housing (which develop small leaks over time) and just a general house cleaning exercise under the hood i am back up to an average of ~34mpg. That milage is 80 percent highway @ 75mph. i have seen the fluctuations you are talking about and it is always attributable to how level the car is when i fuel it up. When I use the same pump at the same fuel station i get the same mileage (make one wonder if some pumps are calibrated inaccurately). This has been the most impressive car i have ever owned! 410,000 miles and original alternator, water pump, starter, no transmission work and no rear end work!!!