"New" Jeep and extended oil changes...

I bought the wife a 1999 Jeep Cherokee with the 4.0 six cylinder engine,it has 30K and is in excellent condition.This is her 2nd Jeep.The first one went over 150K using 10w30 dino oil(it was in a bad wreck and was totalled).This one I'd like to use Super tech synthetic along with a Super Tech filter.Normally I change dino oil at 3mo/3000 mi interval,but would like to extend the change interval when using the synthetic.Comments/suggestions??
You might want to spend some money on analysis to determine how the oil holds up in your rig. If you want to take it for conservative drains like 5000-7500 you can probably do that without sampling everytime once you have a baseline. But first you need to get a baseline to figure out what is safe. I would recommend sampling at 3000 and 5000, 7500, 10000 for your fist interval. For around $80 you can determine what drain interval you want to stick with. Then you wouldn't necessarily need to test it nearly as frequently for future drain intervals. If you decide like I have to only go 5000 miles with synthetic, then you can get away without analysis at all. If you don't want to do analysis, then I wouldn't go over 5000 miles. Just my $.02