A transformer oil basestock is disclosed that includes at least 99 wt % of naphthenes and paraffins, based on the total weight of the transformer oil basestock, wherein the weight ratio of naphthenes to paraffins is at least 1, as measured by GC-MS, and wherein the paraffins consist essentially of isoparaffins, as determined by GC-FID. In addition, a transformer oil composition is disclosed that includes the transformer oil basestock, an anti-gassing agent and an antioxidant.

A grease composition may be used for speed reducers and speed increasers, being excellent in both torque transmission efficiency and leak prevention performance. The grease composition may contain a base oil (A) and a urea-based thickener (B), which is used for speed reducers and speed increasers, wherein the particles containing the urea-based thickener (B) in the grease composition satisfy the following requirement (I). Requirement (I): the area-based arithmetic average particle diameter of the particles is 2.0 μm or less, as measured by a laser diffraction/scattering method.

A polyalkylene glycol-based compound of formula (1): ##STR00001##
may be one in which R.sup.1 is a monovalent hydrocarbon group having 1 to 32 carbon atoms, a monovalent aromatic hydrocarbon group having 6 to 42 ring carbon atoms, a monovalent acyl group having 2 to 32 carbon atoms, or a monovalent oxygen-containing hydrocarbon group having 2 to 32 carbon atoms; R.sup.2 is a monovalent hydrocarbon group having 1 to 32 carbon atoms, a monovalent aromatic hydrocarbon group having 6 to 42 ring carbon atoms, a monovalent acyl group having 2 to 32 carbon atoms, a monovalent oxygen-containing hydrocarbon group having 2 to 32 carbon atoms, or a hydrogen atom; R.sup.3 is a divalent hydrocarbon group having 4 carbon atoms; R.sup.4 is a divalent hydrocarbon group having 2 or 3 carbon atoms; m and n are respectively numbers between 1 and 40 and 0 and 20; and m/(m+n)0.5.