If your engine requires a break-in period, I'd change once the rings have completely seated (2500 miles?). Change it again at 5000 miles and every 5000 miles after that. If the car came with the synthetic alreay in the car, I would assume the factory breaks-in the engine on a stand before installing the engine.

The only reason to start off with dino oil is that synthetic can do too good a job and hinder the abilty of the rings to seat. On a fresh rebuild, do as your builder suggests, and if you rebuilt it yourself, I'd start with dino and then switch.

My Ranger has 135,000 miles on it, and it doesn't leak, doesn't burn oil, and looking into the valve train area, it's as clean as a baby's bottom. When the dipstick is pulled, the oil at the end of a 5000 mile cycle looks almost as good as the new oil going in.

I've used nothing but Mobil 1 (5W-30) and have never had a problem. I'm sending the oil out to be anylized at 150,000 to see what the microscope can see. While it costs 3 times the cost of dino oil, I just consider it cheap insurance. And yes, I'll use it in my Studebaker (10W30).