A heavy hydrocarbon composition with utility as a heavy hydrocarbon base stock comprising at least 95 wt % paraffin molecules, of which at least 90 wt % are isoparaffin, containing hydrocarbon molecules having consecutive numbers of carbon atoms, is a liquid at 100.degree. C., at which temperature its kinematic viscosity, as measured by ASTM D-445, is above 8 cSt, has an initial boiling point of least 850.degree. F. (454.degree. C.) and an end boiling point of at least 1000.degree. F. (538.degree. C.), wherein the branching index (BI), as measured by the percentage of methyl hydrogens, and the branching proximity (CH.sub.2>4), as measured by the percentage of recurring methylene carbons which are four or more carbon atoms removed from an end group or branch, of said isoparaffinic hydrocarbon molecules, are such that: (a) BI-0.5(CH.sub.2>4)<15; and (b) BI+0.85(CH.sub.2>4)<45; as measured over the heavy hydrocarbon composition as a whole.