Rebuilt Tranny Break In

It totally depends on you I personally like synthetic and depends on how long you plan to drive (own) it? Ask the rebuild people what they thing also ask about a proper break in period and procedure.

There shouldn't be any break in needed. The break in procedure after the rebuilds I have done for my transmissions is to run it up and down through the gears at low throttle to make sure all the pistons and accumulators are full of fluid. After that, check the fluid level when it reaches operating temperature. If it's full and ready to go, I take it right to full throttle. If it fails, it was bound to fail anyway and was not done correctly.

So far I've rebuilt the transmission for all the vehicles I have owned and only once had a problem that showed up when I got to the full throttle test. Of course I had to tear it back apart and fix the mistake I made, but it's better to find it first thing than on a road trip. Likewise you're going to be under some sort of warranty with a shop if it has a problem in the first 2 days than if it takes 6 months for the problem to show up.

No warranty on the transmission, it was a fresh rebuild included in the deal when I bought the truck back in July. I decided to use the NAPA Dex 3 Mercon and it's shifting great with no leaks or other issues. After 3 days of light driving I took the truck in for oil change and lube and had the shop check it over for leaks. They said all is good and found no issues so she's ready for the Winter. Thanks for the responses guys

The dex3 should be fine for your tranny, however, I would have probably gone with a quality Dex6 synthetic. It is supposed to be fully backwards compatible and is superior fluid.
Good luck with the new transmission and glad to hear you are back on the road.