I know a lot of guys who honestly believe that using synthetic oil and changing it every 3k is going to make their engines last longer than if they used dino oil, but they are mistaken. Perhaps a few years ago this might have been true, when dino oil was not as good as it is now, but with the new SL ratings, dino oil is getting so much closer to synthetic, at considerably less price.
The main benefits of synthetics are the ability to last for longer drain intervals, as well as flowing faster in extreme cold. So if you're going to do 3k intervals, then it's only real benefit is the better cold weather performance. But even then dino oil is getting better, as I've seen some pretty decent pour point numbers from some dino oils lately. And if your area never gets cold, there goes that benefit too.
Why change it every 3k though? Why not go to 4 or 5k intervals? I'm sure dino oil could go that long now. As a matter of fact, I'd really like to see some oil analysis results on a dino oil just to see how it holds up. It seems like all the analysis results I see are for full or semi synthetics, but none on a $1.50 dino oil.
And for a vehicle that isn't driven much, you definitely want something with moly in it.