Here's something that might surprise a few people --- it did me!
Viscosity measurements following the KRL Shear Stability Test revealed that seven gear lubes sheared down below the minimum viscosity requirements (orange), failing the shear stability requirements of the SAE J306. The two gear lubes with the largest viscosity loss, as reflected in the following graph, were Royal Purple, losing 40.6% of its viscosity, and Torco SGO Synthetic, losing 35.2% of its viscosity. Royal Purple was the only gear lube to fail both the initial viscosity require- ments and the shear stability requirements. It started out too thick and ended up too thin. Torco SGO Synthetic, which had the highest VI in the previous graph, finished the shear stability test as the thinnest of all the oils at 9.97 cSt, far below the minimum 13.5 cSt requirement. Lucas 75/90 Synthetic, with an initial viscosity that exceeded the maximum requirements by 20.8%, passed the shear stability test, but lost 34.5% of its viscosity, the third largest loss of viscosity. Both OEM gear lubes, GM and Mopar, failed the minimum viscosity requirements after the shear test. Of all the gear lubes tested, half did not meet the SAE J306 shear stability requirements.
AMSOIL Severe Gear 75W-90 was in the proper initial viscosity range and retained the highest viscosity after the shear test with a viscosity of 16.03 cSt – the mid-point of the SAE 90 viscosity grade.
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