A composition of compressor working fluid including a dissolved gas or a condensate from a dissolved gas, and a base fluid including at least a first base stock and a second base stock, wherein the viscosity difference between the first base stock and the second base stock is greater than 125 cSt, Kv 100° C. A composition of compressor working fluid including a dissolved gas or a condensate from a dissolved gas, and a base fluid including a first base stock and a second base stock, wherein the viscosity difference between the first and second base stock is greater than 40 cSt, Kv 100° C., and wherein the first and second base stocks are each polyglycol or polyalkylene glycol base derived from propylene oxide. A method of achieving favorable viscosity index for a compressor working fluid. A method of reducing dilution from gas being compressed for a compressor working fluid.