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Transmission Fluids
Background & Properties
Important Transmission Fluid Properties
July 2017
Viscosity index (VI), or the measure of change in an oil's viscosity relative to a change in temperature, is another critical
factor in base oil and finished lubricant performance. Transmission fluids undergo fluctuations in temperature and are
responsible for removing heat from the system. Base oils with high VIs (over 100) experience less change in viscosity
related to temperature. In addition to stay-in-grade performance, high VI base oils offer excellent pumpability and
lower oil consumption.
An exceptional additive package is just as important as a high quality base oil. Differing friction characteristics of
transmission clutch surfaces require a delicate balance of friction modifiers and other additives. Additives typically
found in a premium-performing transmission fluid are:
• Dispersants for sludge and varnish control;
• Anti-oxidants to control oil thickening and breakdown;
• Anti-wear agents to protect planetary gears, bushings, thrust washers, sprags and pumps;
• Seal swell agents to control swelling, hardness and tensile strength of elastomers;
• Corrosion inhibitors to prevent corrosion of bushings and thrust washers;
• VI improvers to minimize viscosity change due to temperature and control sludge and varnish; and
• Friction modifiers to help clutch plate/band friction engage properly and promote
smooth shifts.
Hydrostatic Transmission Fluids (HTFs)
HTFs are charged with protecting not only the transmission and final drives but the wet brake system as well. On the
transmission side, HTFs have a higher coefficient of friction to handle heavy loads, but some friction modifier has been
added to the fluid to prevent squawk and shudder in the wet brake systems. HTFs have a low static coefficient of
friction because the fluids must have some slippery characteristics in order to protect the wet brake system. The chart
illustrates this fact.
Hydrostatic Transmission Fluids
Sliding Speed Decreasing
Torque,
Nm
D 3
