The disclosure relates to viscosity improver concentrates comprising a bio-based liquid decarboxylated rosin acid as a solubilizer, and a viscosity index improver. The viscosity improver concentrate is used in lubricating oil compositions comprising base oil and optional additives. The decarboxylated rosin acid component has at least one or more of: one or more C═C groups, and 40-100 wt. % of tricyclic compounds having 18-20 carbon atoms; a sum of tricyclic compounds as aromatic and cycloaliphatic of 50-100 wt. %, based on total weight of the decarboxylated rosin acid; and an amount of cycloaliphatic compounds of 15-100 wt. %, based on total weight of the decarboxylated rosin acid. The viscosity improver concentrates comprising the solubilizer have comparable properties, including compatibility, kinematic viscosity, and viscosity index, to other commonly used fossil based solubilizers.