Just changed the oil in my diesel Pro 4X at 6k miles and intend on doing it every 6k. Like everyone said, suit your taste/needs. I for one cannot see using motor oil for more than 6k miles in the blistering summers we have here in Central Texas.
For what it's worth, changing the oil on this truck was the easiest I've ever done on a truck - so 6k frequency does not bother me the slightest. (Comparing to changes on Silverados and RAMs - I've never had a shop change oil on my vehicles in 18 years)
I used the OEM oil filter ($9) and T5 Shell Rotella Synthetic Blend 10w30 ($14/gal. At O'Reilly's).
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Titan XD Pro 4X