I just use Delvac 1 and change it every 6 months. That works out to somewhere between 3-4 thousand miles.
603's have loud lifters, they really like clean oil.
Synthetic is nice because it isn't really affected by temperature, so at 0 your engine will spin over much, much faster. I honestly don't see the point of using 15w40 in the winter when you can get that Rotella 5w40 so cheap. Unless you just enjoy hearing that engine turn over nice and slow.
Or during the summer because Rotella 5w40 is rated for year round use.