Abstract: In a central system fluid composition comprising (1) 70 to 95% by weight of a hydrocarbon base oil and (2) 5 to 30% by weight of a viscosity index improver, the improvement wherein said hydrocarbon base oil (1) comprises (a) 25% by weight to less than 50% by weight, based on the weight of the hydrocarbon base oil (1), of an oligomer of 1-decene having an average molecular weight of 200 to 600 and (b) more than 50% by weight to 75% by weight of a petroleum lubricating oil fraction, and said viscosity index improver (2) is a polymethacrylate having a viscosity average molecular weight of 10,000 to 700,000 obtained by polymerizing at least one ester of a saturated monohydric aliphatic alcohol having 1 to 18 carbon atoms with methacrylic acid.