1998 E300 TD w/129k Averaged 30 mpg for the last 10k miles. Changed the fuel lines to 10mm plastic and viton, added coolant heat exchanger to heat fuel and mileage dropped to 21 mpg. Just finished installing Elsbett kit with second filter and proper re-routing of fuel lines and mileage has stayed around 20-21 no matter how I drive. 250 mile trip last weekend with extended freeway driving netted 22 mpg.
Just replaced water pump/thermostat (broken impeller) and radiator. Car has all viton fuel lines and no leaks anywhere. Engine is dry. New air filter. New synthetic (Amsoil) oil and filter. New MB coolant. New drive belt. Just ran two tanks of straight WVO and mileage is still 21 mpg.
Meanwhile throughout all of this the power of the car has been Oustanding! Extremely fast and powerful easily outrunning most cars on the road. In fact the power output has increased while the mpg has dropped.