The invention provides a method for selectively alkylating a diamondoid compound comprising reacting at least one .alpha.-olefin containing at least six carbon atoms with at least one diamondoid compound in the presence of an acid catalyst selected from the group consisting of AlX.sub.3 and BX.sub.3 wherein X is a halogen, in the absence of added proton-donating catalyst promoter. The invention further provides a lubricant composition comprising alkyl-substituted adamantanes wherein the ratio of linear to branched alkyl substituents is at least about 4:1 and wherein the average number of alkyl substitutions per diamondoid molecule is from about 1.5 to about 4.