I use synthetic for two reasons, extended oil change intervals (I don't have time) and I believe it flows better at low temp (-36*F a couple weeks ago) but think it comes down to $$$ you want to spend. In mild/normal driving I would put limit at about 5000miles with cheap oil and filter without causing any pre-mature engine problems $20. - or spend $40 for syn every 3000mi - diminishing returns on you $$$ spent, but they are your $$$.
I've had several trucks that went well over 100k and I used Castrol 10-30. This is the first truck I've ever used synthetic oil in, I waited until 7k then switched to mobil1. The only reason for the change was that I had to drive over 3,000 miles per month and didn't want to change the oil every month. You pay double for synthetic oil and get double the miles between changes, sort of a two for one on the labor.
It doesn't matter which you choose, it matters that you change your oil and filter regularly. It's also important that you use quality oil and filters.