I use what the dealer recommended which I believe is a part synthetic oil. The last time I had it changed the dealer changed it. The right type of oil & liquids made for the engine along with regular maintenance is the life of the engine, IMHO.
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2011 GMC 4X4 dually CC, 6.6 Duramax with Allison Transmission. Formally 2001 Montana,2007 3400RL Montana, presently 2018 3401RS Alpine.
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