You have to run heavier lube in these things especially if you tow and are in a warmer climate. It's that simple.
I think the only reason they run synthetic oil is because they can claim they get a tiny percentage increase in fuel economy which gives them points with the EPA.
$12 a gallon for my rear diff oil (85/140), change every 30-36k miles. 358k miles on my oem diff seal, still dry.
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