I recently switched to Amsoil in all the pertinent areas. Just last week I had the dealer repair a leaking inner CV boot and am now wondering if they had to change the diff. fluid to make the repair.

Any reason to worry going from synthetic to dyno. if they did have to change the fluid because I didn't think to tell them to put Amsoil back in

 

When I swapped diffs (housing and all) I drained the fluid. However, I only did so because I had to wiggle the stupid thing(s) out from under the rig. If I was just changing a CV, my understanding is that a small amount of fluid will leak, but not much. Therefore, I would think the dealer would not drain the fluid. Besides, it should say on the receipt. Did you check that or call them and ask?

As for your second question, no, it will not be a problem to switch from synthetic back to regular.