The invention provides a fluid composition for a fluid coupling, which is excellent in viscosity stability and torque stability, and comprises a polyorganosiloxane base oil having a viscosity of 3,000-500,000 mm.sup.2 /sec at 25.degree. C. and at least one 5-membered heterocyclic compound incorporated in a proportion of 0.01-3.0 wt. % based on the total weight of the composition, said 5-membered heterocyclic compound being selected from the group consisting of thiadiazole derivatives and thiazole derivatives, both, having at least one monovalent group represented by the formula --S.sub.x --R.sup.6 in which R.sup.6 is a saturated or unsaturated monovalent group or atom composed of at least one atom selected from a carbon atom, a hydrogen atom, an oxygen atom, a nitrogen atom and a sulfur atom, and x is a number of 1 or greater.
As a viscous fluid (F) contained in a vehicular heat generator provided with a rotor (33), a non-Newtonian fluid having an apparent viscosity that decreases as the shear rate of the rotor (33) increases (e.g., a kind of silicone oil) is employed. The nominal viscosity of the viscous fluid (F) is in the range of 10,000 cSt to 200,000 cSt. If such viscous fluid is employed, the viscous fluid (F) maintains its shear heat generating function over an extended period even under circumstances where the fluid (F) is subjected to over-shearing by over-rotation of the rotor (33). In addition, low-temperature starting of the rotor is facilitated.
An organopolysiloxane composition for viscous fluid couplings permits little torque variation even when used at a high temperature for an extended period of time. The organopolysiloxane composition comprises an organopolysiloxane liquid with a viscosity of 100 to 1,000,000 sq. mm/s and a platinum compound in an amount such that the amount of platinum metal relative to the liquid is 0.1 to 1,000 ppm. The organopolysiloxane liquid is typically a trimethylsiloxy-endblocked polydimethylsiloxane.
Compositions comprising (a) an oil of lubricating viscosity; (b) 2,5-dimercapto-1,3,4-thiadiazole (DMTD), a derivative of DMTD, or mixtures thereof; (c) a friction modifier; and (d) a dispersant, are useful for lubricating a transmission having a plurality of wet clutches and a plurality of partial power transmission shafts, wherein shifting of gears occurs by a process comprising synchronization of an engaged and a non-engaged partial transmission shaft and engagement of a wet clutch.
