Nice Link on Oil
 

There's a lot of information in there, but I think I'm still going to change my oil every 3000 miles. Just for "peace of mind".

I stopped changing my oil every 3k a long time ago. Its just not needed. i go 5-7,500 for changes.

The reason for the reduced Zinc and Phosphorous levels are not entirely because of the cats. Think about, the levels of this additives would have been gone by the 80's but only now have they been reduced. The real reason is because Detroit went to roller cammed engines so these wear additives weren't necessary anymore so as a result, they started to be reduced. Basically, the oil companies followed what Detroit said.
As for no advantage of using synthetic on a stock engine, not only have I had personally experience, but dynos prove that swapping all of your old stuff for full synthetic (diff, trans, and engine) results in a power gain. It's not much (like 4%) but it shows that it does improve the way your car runs.