All or most small diesel
engines use distillate or gas oil
, larger diesels will often run on light black diesel fuel, these can be anywhere from 500 Kw upwards but running at slower revs 750 to 500 RPM
Its quite possible than an unscrupulous supplier in an out of the way port might have supplied this type of fuel.
Was the fuel initially the normal blue yellow translucent colour, if so then its a contamination problem.
Replacing a tank full of fuel and scrupulous cleaning
of the system will be much cheaper than a fuel injection pump and injector rebuilds
Compare a sample of the black fuel with distillate, filter some through a fine filter paper and look at residue solids.
Check viscosity against distillate, put a drop of each on an angled large plate and compare speed of running down plate, slower running means higher viscosity and potentially an unsuitable fuel for a small diesel.
Just remember the old ad
"Oils aint oils Sol"