Im considering Maxlife ATF in a friends 04 Duramax with allison a-1000 tranny. If I remember correctly, GM changed seal material in about 2006 which made them compatible with Dexron VI. Before then allison recommended C4 and TES-389. This is a more viscous fluid at around 7.6 cst vs maxlife at 6.1. Anyone try Maxlife in an older allison? I would buy some tranSynd, but dont want to drive for an hour to get it. Thanks