Typical acceptable values for continued use of a gear oil are:
Viscosity: AGMA 4 (ISO 150) – greater than 135 cSt and less than 165 cSt at 40°C
AGMA 5 (ISO 220) – greater than 198 cSt and less than 242 cSt at 40°C
AGMA 6 (ISO 320) – greater than 288 cSt and less than 352 cSt at 40°C
Water Content: less than 400 ppm
Total Acid Number (TAN): less than 2.0
Wear Metals: Iron less than 300 ppm
Lead less than 75 ppm
Copper less than 275 ppm
Chromium less than 30 ppm
Aluminum less than 40 ppm
Tin less than 30 ppm
Silver less than 30 ppm
Silicon less than 60 ppm
In addition, if the viscosity increases 10% or more between samples and the TAN increases, the oil should be changed. This is an indication that the oil is oxidizing at a rapid rate.
Note: TAN is the amount of KOH required to neutralize one gram of oil and is a measure of the oxidation. This is sometimes called the neutralization number (NN). Another measure, the oxidation number, is a product-specific value and is performed on an infrared spectrometer. The units for oxidation number are absorbance/cm. To use oxidation number, the oil manufacturer must specify the acceptance limits for the specific product. As an example, Mobil Gear 630 has an upper limit of 8.0.
