A laboratory unit is described with a method for determining the deposit-forming tendencies and oxidation stabilities of lubricants in thin films. The test oil is passed over a spinning aluminum disk heated to temperatures of 500–650 F, which provides a dynamic oil film of about 0.0003 inch thickness. Oxidation stability is determined by measurement of the oxygen consumed. The deposits on the disk are estimated visually. The test can be used for comparative evaluation of base fluids and inhibited fluids.