A method for improving the oxidation stability of an ashless hydraulic fluid or an ashless paper machine oil, comprising: a. selecting a base oil having greater than 90 wt % saturates, less than 10 wt % aromatics, a base oil viscosity index greater than 120, less than 0.03 wt % sulfur, a sequential number of carbon atoms, greater than 35 wt % total molecules with cycloparaffinic functionality, and a ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality greater than 2.1; and b. replacing a portion of an original base oil in the ashless oil with the selected base oil to produce an improved ashless lubricating oil; wherein the improved ashless lubricating oil has a result in the rotary pressure vessel oxidation test that is at least 50 minutes greater than the result in the rotary pressure oxidation test of the ashless hydraulic fluid or ashless paper machine oil.