Hi old_man. I'm actually ?Waldo? on the Samba. 11.25L/100KM is 20.9 miles per US gallon. One thing to note is that winterized fuel gets worse fuel economy by 5-10%. Higher viscosity due to cold weather will also negatively affect overall fuel economy especially from shorter drives. Your 20.1 MPG in the winter might turn into 22+ in the summer without any other changes. IMO, that is still worse that normal. 8L/100KM (29MPG) is better than I have typically seen with a 1.6TD in a Vanagon although within the realm of reason. Round trip no wind highway cruising @ 70MPH with everything in good condition, I typically saw 27MPG with the 1.6TD. That would be in a pop-top camper or high top. A tin top would do better. I am not sure how significant the truck body is to that equation. A TDI will typically do better for fuel economy and power as well.
I am not sure what you mean by pressure regulating bolt. Do you mean the pressure regulator? It is threaded into the pump case, but is not a bolt. The business end is a plunger and spring and the adjustable side is a pressed plug.
Injectors could be affecting the fuel economy as could a tired engine. It wouldn't be a bad idea to have the nozzles replaced and the injectors calibrated. A compression check wouldn't be a bad idea either. As far as other things that can affect fuel economy, there are lots. Here's a non-exhaustive list: low tire pressure, bad tire alignment, imbalanced tire, bent rim, bad CV joints, sticking brakes, worn wheel bearings, incorrect temperature or non-functioning thermostat. I'm sure there's more I haven't thought of.