synthetic always, well almost. Unless, like me you have an Auburn LSD who specs a high quality dino oil...

 

synthetic. amsoil, royal purple, or mobil 1 off the shelf.

 

I'll probably go Royal Purple then

 

You get them gears ordered yet? Go with the ones from Randy's?

 

Think twice and do some research before you run synthetic during the initial break in. I know in the 4x4 world and from when i put 5.29s in my yota when ever you do gear swaps its best to use the cheap mineral stuff so it will wear and break in like its supposed to. After 500 miles of various driving conditions and such, then you should switch to your syn. oil of choice.

 

You know, you may be right, for a ring and pinion swap, I agree 100% and have gone through the procedure for proper break-in many times on here. Didn't even give it a thought with spider gears only...
It wouldn't hurt, and probably wouldn't hurt to ask at Randy's, their gear prices may not be the most rock bottom on the net, but they have VERY knowledgeable people there when it comes to diffs...

 

Another place I would suggest is axleboy.com and ask for Scott. He does great work and can give you good prices. Though I agree Randy's is great place as well and they know their stuff.