Welcome to the forums!
5000 km is roughly just over 3100 miles, so unless he was using it for heavy duty towing a lot, he wasn't using the oil for its full potential (I agree with you).
Some here have gone 12000 miles before on an oil change or more on synthetics. I say why save a few dollars here and there and risk a multi-thousand dollar vehicle? I change mine ever 5000 miles to 6000 miles, depending on winter/summer and if its all been in town or a mix of highway and in town. Town driving is harder on it than highway.
Also, you didn't list what filter you use, I use Wix for my gas V8. Excellent filter, decent price. Mobil1 filters are also good, as are Napa Gold (not regular Napa filters), Purolater, and K&N. Stay away from FRAM and Mopar.
I'd also ask this in the diesel section of the forum, see what they say.
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5.9L 360ci Magnum V8
Single 3" Flowmaster Super 44 Exhaust w/ Magnaflow Catalytic Converter
Volant Cold Air Intake Kit
Fastman 52mm Throttle Body
Modified Intake Manifold w/ Hughes Plenum Reinforcement Kit
SCT XCalibrator 2 Tuning by Hemifever
Awaiting Install: 3.92 gearing all around & Auburn LSD in rear axle
Solutions to Common 2nd Generation Dodge Ram Problems
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