The present invention includes a method for adjusting a fluid density. In
one embodiment, a method for upgrading a petroleum-derived
hydrocarbonaceous fraction comprises providing a synthetically-derived
hydrocarbonaceous fraction, wherein the synthetically-derived
hydrocarbonaceous fraction is derived from synthesis gas, and further
wherein the synthetically-derived hydrocarbonaceous fraction and the
petroleum-derived hydrocarbonaceous fraction have a difference in density
at 15.degree. C. of at least about 60 kg/m.sup.3; and blending both
fractions so as to form a blend suitable for use as a diesel or diesel
blendstock, wherein the blend has a density at 15.degree. C. equal to or
more than about 800 kg/m.sup.3; alternatively or additionally, equal to
or less than about 860 kg/m.sup.3. The blending is also effective in
reducing the sulfur content of the petroleum-derived hydrocarbonaceous
fraction. In preferred embodiments, the synthetically-derived
hydrocarbonaceous fraction is a Fischer-Tropsch diesel.