franchi

I do not have experience with small diesels like yours. My smallest engine is a OS 40FSR and the largest is a ST 4500. In all I have about 15 diesel engines. From my experience black goo is a sign of over compression. I would suggest backing the compression a bit while slowlyclosing the needle until a good power is obtained with slightly grayish exhaust.
As to fuel, I have about 15 gallons of Davis's fuel that are over 20 years old and run like champs. If you have Davis's fuel, my recommendation would be to use it AS IS. Bob knows very well what he is doing.