The significance of used oil analyses as applied to tractor hydraulic oils is discussed with particular emphasis on fluid commingling. Pentane insolubles and changes in TAN or TBN are used to assess in-service oxidation stabilities of tractor transmission-hydraulic oils. Spectrographic wear-metal analyses were difficult to interpret without detailed information on unit design, maintenance and use. A baselining technique can be helpful for evaluating wear in tractor field tests.Estimates of fluid commingling were developed from used oil additive-metal analyses. Commingling was extensive, averaging 43% in seventeen farm tractors of various manufacture. No field problems have been identified that are caused by commingling. Our experience indicated that a common transmission-hydraulic lubricant for service fill use in all farm equipment is feasible.