Isuzu C223 diesel? If so this is an exact copy of a question posted at
http://www.isuzupup.com/viewtopic.php?t=18467 back in 2013 so why is it being asked again on the KJ CRD forum?
I have no clue what oil and/or oil change interval is recommended for that engine. A 3,000 mile oil change interval was a standard back in the days of single weight non-synthetic oils and today is only recommended by those in the business of making money out of frequent oil changes. I suspect you could extend the interval to say 5,000 miles assuming your driving is not limited to short in town distances BUT that's a pretty old engine series so if it's high mileage on the block staying with a short interval might be prudent. An alternative might be to invest in some oil analysis while extending your change interval to see what's going on.