A process is disclosed to produce liquid oligomers of olefins, such as l-decene, with controlled molecular weight and with branch ratios below 0.19 and having higher viscosity indices than oligomers with higher branch ratios. These oligomers with low branch ratios and controlled molecular weight can be used as basestocks for many lubricants or greases with an improved viscosity-temperature relationship, oxidative stability, volatility, etc. They can also be used to improve viscosities and viscosity indices of lower quality oils. The olefins can, for example, be oligomerized in the presence of hydrogen over a supported and reduced metal oxide catalyst from Group VIB of the Periodic Table to give oligomers of controlled viscosity suitable for lubricant application. More particularly, the instant invention is directed to a process for the oligomerization of olefinic hydrocarbons containing 6 to 20 carbon atoms which comprises oligomerizing said hydrocarbon in the presence of hydrogen under oligomerization conditions, wherein the reaction product consists essentially of substantially non-isomerized olefins. For example, alpha olefins such as l-decene, and wherein a major proportion of the double bonds of the olefins or olefinic hydrocarbons are not isomerized, in the presence of a suitable catalyst, e.g., a supported and reduced metal oxide catalyst from Group VIB of the Periodic Table.