Ideally you should change the oil not only at km range but also a time range.
I use the Delvac MX and change the oil every 3000kms or 3 months whichever comes first. Some say that's a bit too early, but I'm running a 1KZ-TE with 160,000kms on the odometer and its still not been touched (touchwood)
Keep in mind that you are running a D4D engine and the diesel fuel here does not support it well.