Interesting google search "The JDM 303 used sperm whale oil to meet the tractor’s specification and other equipment fifty years ago. Because of animal conservation, sperm whale oil is no longer used. Today’s manufacturer uses the 303 Hydraulic Oil as a reference to JDM 303. This is odd because John Deere no longer produces JDM 303."
Also from Yesterdays Tractors forum, "Use John Deere Hy-Gard. That is what replaced the old 303 hydraulic oil, and it meets all former and current specs for hydraulic/transmission oils in JD tractors. At least for everything built since 1960 anyway. The Low-Viscosity Hy-Gard is intended for extremely cold temps and use in hydrostatic transmission systems.