Liquid olefin oligomers are produced by the oligomerization of C.sub.2 -C.sub.5 alpha olefin alone or with ethylene as a co-monomer. The oligomers having high viscosity index and a structure which is characterized by a regio-irregularity of at least 20 percent, usually from 20 to 40 percent. The olefins are oligomerized with a reduced valence state chromium oxide catalyst on a silica support, usually at a temperature from 90.degree. to 250.degree. C., although lower temperatures can also be used with ethylene as a con-monomer. The liquid oligomerization products can be produced in wide range of viscosities including the direct production of low viscosity lubricants having high viscosity index.