When I purchased my E300D, 12 years ago, the fuel gauge was erratic.

After a few hundred miles and a can of Sea Foam in the fuel tank, the gauge has never been erratic again. I use Sea Foam, 2 cycle marine oil, Power Service Diesel Kleen, and Stanadyne mixed together to make my own diesel additive. Typically add about 16 oz to a 20 gallon fillup. Could probably do a better mix but seems to work for me.