I am getting ready to install my 3:73 FMS gears in my 05. Question is when will I need to change the gear oil after the install and should I run non-synthetic as the break-in first or start with synthetic?

 

Use synthetic, that's what Ford calls for in these cars with the limited-slip in there. Use some friction modifier along with the synthetic fluid and you'll be good to go.

 

FWIW, the shop that installed my gears is adamant against using synthetic gear oil. All they do is rear end work (for all kinds of vehicles), and they have a stellar reputation for high quality. See Tech Tip #2 here: http://sdgearaxle.com/_wsn/page4.html