Just my humble opinion:

I've been running 5w-30 since the first oil change. It's what the dealer stocks, and their service manager has a bulletin from Toyota stating 5w-30 is OK for the TCH. I consistantly exceed the EPA mileage ratings despite running 5w-30. I seriously doubt going to synthetic oil will significantly improve the gas mileage, and any increase will not pay for the extra cost in the oil with 5K oil change intervals.

That said; if I lived in an area with serious cold weather, I would seriously consider running synthetic oil due to it's superior cold weather flow characteristics (pour point and pumping point.) I would also consider running synthetic oil if I was planning on extended oil change intervals, say every 15 to 20K miles. In fact the one vehicle I've used synthetic in is my F150 (~240,000 miles on the original engine). Mostly I did it due to the extream heat encountered from towing in 100 to 110F summer heat where I used to live.

Modern dino engine oils already protect well past 200,000 miles. I've driven several Hondas, and one Ford well past 200K on dino oil without any engine problems. Oil usage was never worse than a quart every 2K miles.