Ignore the Suzuki book. The DRZ has been around along time now and it is well known that you really need to change the oil every 2000km with hard riding and maybe 3000km with touring.

I changed mine every 3000km in Africa and the oil was ALWAYS very black and dirty. I always used the best oil I could get and that was always car oil but rated to at least AF-G.

If you set your valve clearances before you go, you should be good for at least 25,000km. Mine were in perfect spec after that mileage still.

As with the above advice. CHECK YOUR OIL often. Every few days or 500 miles. It's a very easy job and only takes a minute.

If you look after a DRZ engine, it will take some serious abuse. Just don't push your luck. It is very easy to work on and very simple. There are no excuses

Just drop the oil and put 2L of oil in it. EASY. Change the filter every 2-3 changes.

RE the clutch.. Mine was starting to slip at about 25,0000 MILES. I don't know if that was down to the oil or not. The plates were in spec but I think probably highly contaminated by car oil (most do have clutch friction modifiers in them) Also, my bike was overloaded and I had the wrong gearing. I suggest 15/42 for best engine life and universal riding.

The MOST MOST MOST important thing on a DRZ400 is to pack REALLY light.

__________________
Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.