When to Switch to Synthetic?

[BigGreen06]

I was just wondering when would be a good time to switch to synthetic oil. I'm just about to go through my factory 36K warranty, but I did get the extended 4yr 100K warranty. I don't want to burn my dealer by changing to synthetic (thinking about Royal Purple (don't know if that is good for the diesel or not)), I have a really good relationship with my dealer and don't want to screw that up. Just thought I'd ask opinions on switching and to see what you guys are running for synth or not. I know that Shell Rotella is pretty big, The only reason for Royal Purple is that I run it in my high mileage offroad truck, as well as trans fluid and rear end fluid and it is great in it, lasted 3-4k at least and still lubricates real well. Its just great at lubricating and keeping things cool.
Thanks.

-BG06.