I've been using the 5W-40 Rotella synthetic in the 603 for the past year.

Changes every 5K. Two oil analysis confirmed that the oil could have easily gone longer. The soot was still below 1% on both oil changes.

On the last change, I extended the interval to 6K. Didn't get it analyzed however.

I'm quite sure you can run this oil to at least 7.5K on a gasser. Probably go even longer than 7.5K, but, I'd sure not even think about anything less.