Diff oil questions

Q1: Is there any reason to avoid synthetic gear lube in an open diff as long as it meets GL5 specs?

Q2: Is there really any difference between the different brands of gear lube (Valvoline, Peak, Mobil 1, Lucas, Royal Purple, etc) other than price within a given viscosity spec?

No reason at all, its actually better, withstands heat better. factory tow packages come with syn oil in them.

just different brands, i run Redline syn 75w140 in my jk, it costs me $100 to do both diffs- i think its worth it.

Awesome. I can get 12 1-qt bottles of Redline on Amazon for $200.

What's the Peak gear lube that PepBoys sells for $6/qt? Is this recycled diff fluid that is resold for cheap?

Some of the cheaper oils have less additives in them, that goes from brand to brand as well. Also most of the cheap oils cannot with stand pressure. That is what breaks the oil down along with heat.