Should be 14 mm hex which you can get at AutoZone. The plugs are usually very tight, especially after 200kmi.

There are quite a few good synthetic gear oils you can get. Most are of the 75W-90 viscosity. Mobile 1, Red Line, etc.

I have used Mobile 1 75W-90 and Redline 75W-90 and 85W-140 and they are all good.