In regards to the gear oil, as long as you're not getting any metal particles in the oil when you change it, then you can start using synthetic. I use the Lucas 75W-90 Synthetic (and have since 100 miles), and don't have any problems.
Just remember to change oil early and often, since the whole point of oil changes on a new engine is to remove any wear particles before they can damage engine parts. I changed at 50, 100, 200 and 400 miles. Next change is at 800 miles, then at 600 mile intervals after that (so, oil change intervals of 50, 50, 100, 200, 400, 600).
Once you're no longer getting any metal particles in the engine oil (look at the oil in a shallow pan, under strong light), you can switch to synthetic.
My engine is at that point now, but I'm going to wait until 1000 miles, just to be sure it's fully broken in.