GM recommends Dexron 6 only. Dexron 3 will work but the service life of the fluid is half that of Dexron 6. Instead of changing it at 50,000 miles under severe usage you would want to change it at 25,000 miles. Also some of the Kodiak and Topkicks with the Allison 1000 used this stuff called Transyn fluid. It was more of an amber color like traditional hydraulic fluid and to me it feels like its a little thicker viscosity.