A working fluid for a heat transfer device comprising: i) a C 1-6 hydrocarbon refrigerant; and ii) a polyol ester lubricant composition having a kinematic viscosity at 40°C of 22-125 cSt, and a viscosity index of greater than 140, and comprising a mixture of compounds of formula I wherein n is an integer of from 1 to 12, each R is independently an alkyl carbonyl of from 5 to 10 carbon atoms, each R 1 is independently selected and is either a group R or a substituent of formula II: wherein at least 80 molar % of all R groups are independently selected from straight chain alkylcarbonyls of from 7 to 10 carbon atoms, and wherein the polyol ester lubricant composition comprises a) from 30 to 55 wt% of a compound of formula I wherein n is 1 and each R 1 is independently selected and is a group R, b) from 1 to 20 wt% of a compound of formula I wherein n is 2 and each R 1 is independently selected and is a group R, c) from 1 to 10 wt% of a compound of formula I wherein n is 3 and each R 1 is independently selected and is a group R, and d) from 25 to 45 wt% of a compound of formula I which are pentaerythritol oligomers of 4 or more pentaerythritol monomer groups, wherein the wt% is based on the weight of all compounds of formula I present in the lubricant composition, wherein the polyol ester lubricant composition further comprises C 5-10 alkylcarboxylate esters of trimethylol propane wherein 75 molar % or more of alkyl carbonyls in the esters of trimethyl polyol are straight chain alkyl carbonyls, and wherein the polyol ester lubricant is present in a ratio of from 1 to 500 parts by weight per 100 parts by weight of the refrigerant. i) a C 1-6 hydrocarbon refrigerant; and ii) a polyol ester lubricant composition having a kinematic viscosity at 40°C of 22-125 cSt, and a viscosity index of greater than 140, and comprising a mixture of compounds of formula I wherein n is an integer of from 1 to 12, each R is independently an alkyl carbonyl of from 5 to 10 carbon atoms, each R 1 is independently selected and is either a group R or a substituent of formula II: wherein at least 80 molar % of all R groups are independently selected from straight chain alkylcarbonyls of from 7 to 10 carbon atoms, and wherein the polyol ester lubricant composition comprises a) from 30 to 55 wt% of a compound of formula I wherein n is 1 and each R 1 is independently selected and is a group R, b) from 1 to 20 wt% of a compound of formula I wherein n is 2 and each R 1 is independently selected and is a group R, c) from 1 to 10 wt% of a compound of formula I wherein n is 3 and each R 1 is independently selected and is a group R, and d) from 25 to 45 wt% of a compound of formula I which are pentaerythritol oligomers of 4 or more pentaerythritol monomer groups, wherein n is an integer of from 1 to 12, each R is independently an alkyl carbonyl of from 5 to 10 carbon atoms, each R 1 is independently selected and is either a group R or a substituent of formula II: wherein at least 80 molar % of all R groups are independently selected from straight chain alkylcarbonyls of from 7 to 10 carbon atoms, and wherein the polyol ester lubricant composition comprises a) from 30 to 55 wt% of a compound of formula I wherein n is 1 and each R 1 is independently selected and is a group R, b) from 1 to 20 wt% of a compound of formula I wherein n is 2 and each R 1 is independently selected and is a group R, c) from 1 to 10 wt% of a compound of formula I wherein n is 3 and each R 1 is independently selected and is a group R, and d) from 25 to 45 wt% of a compound of formula I which are pentaerythritol oligomers of 4 or more pentaerythritol monomer groups, a) from 30 to 55 wt% of a compound of formula I wherein n is 1 and each R 1 is independently selected and is a group R, b) from 1 to 20 wt% of a compound of formula I wherein n is 2 and each R 1 is independently selected and is a group R, c) from 1 to 10 wt% of a compound of formula I wherein n is 3 and each R 1 is independently selected and is a group R, and d) from 25 to 45 wt% of a compound of formula I which are pentaerythritol oligomers of 4 or more pentaerythritol monomer groups,