I'm a courior and I run the Marine 15w40 with the Amsoil filter, I do oil analyis, Since jan I have over 600hrs of run time and approx 25,000kms on the same oil, I usually change it at 30,000kms even though the oil reports come back saying resample at next maint interval (means oils good leave it alone) in that 30,000km span I end up adding a couple of liters.
I think its safe to say you can double your interval that your doing now, then resample the oil BEFORE you change it. Mine runs 8-10hrs a day city stop and go, if you drive your truck half way normal you could likely surpass the interval I'm on.