Refrigerating machine oil

1. A refrigerating machine oil comprising:

a lubricating base oil comprising a mineral oil;

a phosphorus-containing additive comprising at least one selected from the group consisting of triphenyl phosphate, tricresyl phosphate and triphenyl phosphorothionate; and

a polymer selected from the group consisting of:

(I) a copolymer comprising at least one of a first (meth)acrylic acid ester and at least one of a second (meth)acrylic acid ester, as monomer units, the first (meth)acrylic acid ester having a linear alkyl group having 1 to 18 carbon atoms, and the second (meth)acrylic acid ester having a 2-position branched alkyl group having 4 or more and less than 20 carbon atoms, wherein the copolymer of (I) is selected from:

(i) a copolymer consisting of the first (meth)acrylic acid alkyl ester and the second (meth)acrylic acid alkyl ester, as monomer units, and

(ii) a copolymer consisting of the first (meth)acrylic acid alkyl ester, the second (meth)acrylic acid alkyl ester and an α-olefin having 8 to 12 carbon atoms, as monomer units; and

(II) a copolymer consisting of a maleic acid ester and an α-olefin having 8 to 12 carbon atoms, as monomer units, wherein the maleic acid ester is an ester of maleic acid and an alcohol comprising a linear aliphatic alcohol having 4 to 10 carbon atoms,

wherein the polymer has a number average molecular weight (Mn) of 4000 or more and 20000 or less, a weight average molecular weight (Mw) of 8000 or more and 30000 or less, and an Mw/Mn of 1.2 or more and 2.5 or less,

wherein the refrigerating machine oil has a kinematic viscosity at 40° C. of 10 mm 2 /s or less, and is used with a refrigerant, and

wherein

a content of the lubricating base oil is 90% by mass or more,

a content of the phosphorus-containing additive is 0.1% by mass or more and 3% by mass or less, and

a content of the polymer is 0.5% by mass or more and 5% by mass or less, based on the total amount of the refrigerating machine oil.

2. The refrigerating machine oil according to claim 1 , wherein the lubricating base oil comprises a first hydrocarbon base oil having a kinematic viscosity at 40° C. of less than 6 mm 2 /s.

3. The refrigerating machine oil according to claim 2 , wherein the lubricating base oil is a mixed base oil of the first hydrocarbon base oil and a second hydrocarbon base oil having a kinematic viscosity at 40° C. of 6 mm 2 /s or more, and

a (A)/(B) ratio is more than 1 and 1.5 or less, wherein (A) is a kinematic viscosity at 40° C. of the mixed base oil, and (B) is the kinematic viscosity at 40° C. of the first hydrocarbon base oil.

4. The refrigerating machine oil according to claim 1 , wherein the polymer is the copolymer consisting of the first (meth)acrylic acid alkyl ester and the second (meth)acrylic acid alkyl ester, as monomer units.

5. A working fluid composition for a refrigerating machine oil comprising:

the refrigerating machine oil according to claim 1 ; and

a refrigerant.