If you are using syn you can go more than 3k if you want. I use a semi-syn and change every 3k but thats just me. There was a thread on here where guys were going much longer between changes. It sounds like a pretty expensive oil change, so I would let go a little longer if you are comfortable doing so.
91XJ- NP231, 31x10.5 MTR's, 15x8 soft 8's, BB, K&N intake, Cherry bomb exaust, CB, Stereo mods, fog lights, custom Aluminum body work to cover rust.
Rember: If they dont find you handsome, at least they will find you handy!