I love these cars. My current one is a 1993 6 cyl W124 300D. Brought directly from Germany to New Zealand and I am the second owner. Never old in the US. Non turbo.
I have run 100% biodiesel for the last 2 + years totally unaltered. No modifications whatsoever. I change the fuel filter every 10,000 kms and that is all in nearly 200,000kms. My mileage is simply amazing. The gearing is different in this vehicle designed for long haul. Factory. My best long haul is just under 47mpg and around town approx 32 mpg. No repairs just filters and fluids.