I agree with BigWaylon. The syntheic might dissolve some sludge around the seals and then you could start leaking oil. Now, thats not necessarily a bad thing cause then you would know what seals, etc needs replacing, but its up to you. I use Amsoil synthetic in all my vehicles, but I've had them since they were new.
00 Jeep Wrangler Sport 4.0L - 2" BB, 2"BL, 33's, 4.56 gears, Auburn in rear
Pics of my Jeep http://community.webshots.com/album/76582748MYTDqR