A process for producing an API Group I base oil, comprising: blending a lower quality base oil that does not meet API Group I specifications with a Fischer-Tropsch derived distillate fraction having defined pour point, viscosity index and Oxidator B, and isolating an API Group I base oil that has improved defined properties. A process for producing an API Group I base oil, consisting essentially of: (a) selecting a lower quality base oil not meeting API Group I specifications, having defined saturates, viscosity index and Oxidator BN; and (b) blending the lower quality base oil with a Group II base oil and a Fischer-Tropsch derived base oil. A process for improving the lubricating properties of a lower quality base oil. Also, a process for operating a base oil plant.