NOVELTY - Formation (M1) of hydrocarbon product involves converting synthesis gas in Fischer-Tropsch reaction to hydrocarbon products (p1); and hydrotreating (p1) to obtain hydrotreated hydrocarbon products (p2) having n-paraffin (at least 50 wt.% of (p2) and branched paraffin). The branched paraffins are monomethyl branched paraffins. The ratio of end-chain monomethyl branching to internal branching of the monomethyl branched paraffins is at least 1:1.5.    USE - For the formation of hydrocarbon product e.g. at least one of chemical naphtha for 5-9C ethylene cracking, fuel cell 4-9C feedstocks, normal 5-30C paraffin compounds, 5-30C isoparaffin compounds, normal 8-25C paraffin and 8-25C isoparaffin drilling fluids, lubricating 5-30C fluids, drilling 5-30C fluids, food grade 5-60C solvents, base 5-60C oils, 5-60C solvents, 5-60C oils and functional 5-60C fluids (all claimed), paraffinic, isoparaffinic, cycloparaffinic and aromatic hydrocarbons.    ADVANTAGE - The alkylaromatic sulfonate product has at least one of a biodegradability of at least 90% as determined by ASTM D2667-95(2001), a Kraft temperature of at most 50 degrees C or a loss of alkylaromatic sulfonate product as surfactant in hard water of at least 40 or 50%. The hydrotreatment facilitates the conversion of many oxygen-containing hydrocarbons to paraffins. The method does not use costly molecular sieve adsorbents and equipment. The method is efficient and is practiced commercially.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) formation (M2) of a hydrocarbon product involving:    (i) converting synthesis gas in a Fischer-Tropsch reaction to (p1);    (ii) hydrotreating (p1) to obtain (p2) having an initial n-paraffin content of at least 67 (preferably at least 80, especially at least 90, particularly at least 94) wt.% of (p2) and having branched paraffins; and    (iii) separating at least a portion of at least one non-linear branched paraffin isomer from (p1) through distillation to obtain an n-paraffin product having an n-paraffin content percentage by weight of the n-paraffin product that is greater than the initial n-paraffin content (preferably at least 0.005 wt.%); and    (2) treatment (M3) of a hydrotreated Fischer-Tropsch (F-T) product in the 5-24C range involving:    (a) separating a lightest isomer paraffin and a heaviest isomer paraffin through distillation to obtain selected range of intermediate paraffins;    (b) separating at least one of a light n-paraffin isomer and a heavy n-paraffin isomer from the intermediate paraffins to obtain monomethyl isomer-rich stream, which includes at least one of a monomethyl isomer of one carbon more than the light n-paraffin isomer and a monomethyl isomer of the same carbon number of the heavy n-paraffin isomer, and a remaining intermediate paraffin stream from the intermediate paraffins through distillation;    (c) repeating separation of at least one of a n-paraffin and monomethyl isomer-rich stream of step (b) from the remaining intermediate paraffin stream at least one time; and    (d) combining the n-paraffin isomers separated in steps (b) and (c) with the remaining intermediate paraffin stream to make a high n-paraffin product; and/or combining the monomethyl isomer-rich streams separated in steps (b) and (c) to make a high monomethyl isomer-content product