Used Gear Lube Analysis for 1992 Suburban's Rear Differential. Differential had 254,763 miles and 73,687 miles on Gear Lube. This is a synthetic Gear Lube, 75W90. Physical Properties: Viscosity 14.5 [Still in SAE 90 grade]. % Solids TR Oil Degradation: % Oxidation 96.4% out of 100% %NOx 57.3% of 100% TAN 3.6 SpectroAnalysis: Fe - 231 Cu - 1 Sn - 1 Al - 2 Ni - 1 Mn - 3 Si - 109 Na - 2 P - 1152 OAI Comments: Oxidation elevated - No Kidding! TAN Elevated - No Kidding! Analyze Operating Conditions - This means "open-er-up" and see what's going on. Change OIL, Resample at Next Regular Interval. All other elements zero. Analysis didn't include sulphur. Would have liked to have seen the Sulphur content. In my view, they should include this for gear oils. When the metallurgical analysis comes in I will reveal the rest of the story. In the mean time, consider what oxidation means in this context and why the Total Acid Number "TAN" was high.