This invention relates to a low-viscosity lubricating oil and functional fluid compositions, comprising: a major amount of an oil having a kinematic viscosity of up to about 4 cST at 100.degree. C.; and a minor antiwear amount of (A) a compound represented by the formula R.sup.1 R.sup.2 N--C(X)S--(CR.sup.3 R.sup.4).sub.a Z (A-I) wherein in Formula (A-I), R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently hydrogen or hydrocarbyl groups, provided that at least one of R.sup.1 and R.sup.2 is a hydrocarbyl group; X is O or S; a is zero, 1 or 2; and Z is a hydrocarbyl group, a hetero group, a hydroxy hydrocarbyl group, an activating group, or a --(S).sub.b C(X)--NR.sup.1 R.sup.2 group wherein b is zero, 1 or 2; provide that when a is 2, Z is an activating group; and when a is zero, Z can be an ammonium, amine or metal cation. In one embodiment this composition further comprises (B) a phosphorus compound.