The manufacturer doesn't know how you'll drive the car or in what conditions, so they are recommending a conservative interval. The oil manufacturer wants to convince you that the oil is very good, so they are recommending a less conservative interval.
The best approach is to do a UOA, the next best is to take your conditions into account, and UOAs of similar engines/oils/conditions here, and make a reasonable guess.
I would personally consider 3k to be overly conservative under almost any conditions, and 5k or so more reasonable on dino.
- Glenn