What is disclosed are methods for making polymer-in-oil solutions, useful for improving the viscosity-temperature relationship and low-temperature properties of lubricating oils when added thereto, which methods comprise polymerizing a methacrylic acid ester of an alcohol having 8 to 18 carbon atoms onto a polyolefin polymer of an olefinic hydrocarbon monomer having 2 to 4 carbon atoms in an oil solution of the polyolefin and ester monomers, then adding further polyolefin polymer to the solution and finally graft copolymerizing a polymerizable heterocyclic compound having a basic nitrogen atom in the ring thereof onto the resulting mixed polymers as backbone polymers