Which brand of def fluid.
Was talking to the commercial truck guy at a ram dealer and he said they recommend using mopar brand def fluid in the cummins hd diesel.true or false.
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Your manual, and I assume this is true for the Powerstroke and Duramax as well, just calls for API certified DEF. Walmart Super Tech is API certified and runs a bit over $8 per 2.5 gallon jug, and I've used it without issues - it's more convenient for me than driving across town to get it at a pump. I don't see a reason to pay more for the name brand stuff - it probably comes from the same factory anyway.
