Ultra-fluid lubricating composition

1. A lubricant composition of a grade according to the SAEJ300 classification defined by the formula (I)

(X) W (Y)   (I)

wherein

X represents 0 or 5;

Y represents an integer ranging from 4 to 20; and comprising

(a) at least one polyalphaolefinic oil (PAO) for which the kinematic viscosity measured at 100 C according to the ASTM D445 standard ranges from 1.5 to 8 mm 2 .s −1 ;

(b) from 10 to 80% by weight of the composition of at least one oil of group V for which the kinematic viscosity measured at 100° C. according to the ASTM D445 standard ranges from 1.5 to 8 mm 2 .s −1 ;

(c) at least one comb polymer for which the three pendant chains comprise at least 50 carbon atoms and selected from among the copolymers of at least one polyolefin and of at least one poly(alkyl)methacrylate; the copolymers of at least one polyolefin and of at least one poly(alkyl)acrylate;

wherein the at least one oil of group V is selected from among

a monoester of formula (B1);

wherein

R 1 represents a linear or branched, saturated or unsaturated hydrocarbon group, comprising from 14 to 24 carbon atoms;

R 2 represents a linear or branched, saturated or unsaturated hydrocarbon group, comprising from 2 to 18 carbon atoms; and

a polyalkylene-glycol (PAG) of formula (B2)

wherein

R3 represents a linear or branched C1-C30-alkyl group;

m and n represent independently an average number ranging from 1 to 5; and

wherein the weight ratio (a/b) between the polyalphaolefinic oil (PAO) (a) and the oil of group V (b) ranges from 0.1 to 4.

2. The lubricant composition according to claim 1 of a grade according to the SAEJ300 classification selected from among OW4, OW8, OW12, OW16, OW20, 5W4, 5W8, 5W12, 5W16, 5W20.

3. The lubricant composition according to claim 1 , for which the kinematic viscosity measured at 40° C. according to the ASTM D445 standard ranges from 12 to 30 mm 2 .s −1 .

4. The lubricant composition according to claim 1 for which the kinematic viscosity of the PAO, measured at 100° C. according to the ASTM D445 standard, ranges from 1.5 to 6 mm 2 .s −1 .

5. The lubricant composition according to claim 1 for which the average molecular mass by weight of the PAO ranges from 50 to 500 Da.

6. The lubricant composition according to claim 1 comprising from 20 to 80% by weight of the oil composition of group V.

7. The lubricant composition according to claim 1 , for which the comb polymer for which the pendant chains are obtained by polymerization or by copolymerization of olefins.

8. The lubricant composition according to claim 1 , for which the comb polymer is prepared by copolymerization

of an olefinic monomer of formula (C2)

wherein

Q 3 represents a linear or branched C 2 -C 8 -alkyl group; a C 3 -C 8 -cycloalkyl group; a C 6 -C 10 -aryl group;

Q 4 represents a hydrogen atom or a methyl group;

L 1 represents a residue of 1,4 addition of butadiene; a residue of 1,4 addition of butadiene substituted with at least one C 1 -C 6 -alkyl group; a residue of vinyl addition of styrene; a residue of vinyl addition of styrene substituted with at least one C 1 -C 6 -alkyl group;

L 2 represents a residue of vinyl addition of butadiene; a residue of vinyl addition of butadiene substituted with a C 1 -C 6 -alkyl group; a residue of vinyl addition of styrene; a residue of vinyl addition of styrene substituted with at least one C 1 -C 6 -alkyl group;

k and j represent independently 0 or an integer ranging from 1 to 3,000 and the sum (k+j) is equal to an integer ranging from 7 to 3,000; and

of an alkyl-acrylic ester monomer or alkyl-methacrylic ester of formula (C3)

wherein

Q 5 represents a hydrogen atom or a methyl group;

Q 6 represents a linear or branched C 1 -C 26 -alkyl group.

9. The lubricant composition according to claim 1 for which the comb polymer is prepared by copolymerization

of an olefinic monomer of formula (C4)

wherein

Q 7 represents a C 1 -C 6 -alkyl group; a C 6 -aryl group;

Q 8 represents a hydrogen atom or a methyl group;

Q 9 , Q 10 and Q 11 represent independently a hydrogen atom or a C 1 -C 18 -alkyl group;

x, y and z represent independently 0 or an integer ranging from 1 to 3,000 and the sum (x+y+z) is equal to an integer ranging from 7 to 3,000; and

of a alkyl-acrylic ester monomer or methacrylic- alkyl ester of formula (C3)

wherein

Q 5 represents a hydrogen atom or a methyl group;

Q 6 represents a linear or branched C 1 -C 26 -alkyl group.

10. The lubricant composition according to claim 1 , for which the VI is greater than 220.

11. A method for the lubrication of a vehicle engine or for improving the lubrication of a vehicle engine

during an urban cycle according to the European cycle NEDC; or

during an urban cycle according to the European cycle WLTP; or

during a cold cycle according to the European cycle ECE; comprising a step of applying the lubricant composition according to claim 1 to said vehicle engine.

12. The method according to claim 11 for lubricating an engine of a hybrid vehicle.

13. A method for improving the Fuel Eco (FE) of a lubricant or for reducing the fuel consumption of an engine, comprising a step of adding the lubricant composition according to claim 1 into said lubricant, or a step of applying the lubricant composition to said engine.