What is disclosed is a magnetorheological fluid useful for incorporating within the working gap of magnetorheologically controllable linear or rotary devices. The MR fluid includes a carrier fluid component and a magnetic responsive particle component characterized by D.sub.10 of 2 .mu.m up to and including a D.sub.10 of 5 .mu.m, a D.sub.50 of 8 .mu.m up to and including D.sub.50 of 15 .mu.m; a D.sub.90 of 25 .mu.m to and including a D.sub.90 of 40 .mu.m; and further characterized by least squares regression (R.sup.2) particles size against log normal cumulative volume percent of greater than or equal to 0.77. Optional preferred additives included therewith include thixotropic agent or viscosity modifier, dispersant or surfactant, antioxidant, corrosion inhibitor, and one or more lubricants.