some brands number their diff & shock oils differently. I have a mugen shock oil thats says 350wt when infact its actually 35wt. The 50000 may infact be 5000 and its just the way the manufacturers term it. But if it really is 50000, then yes that would be your problem. But that sounds way way way too thick going by normal values so I doubt the wheels would even move using oil that thick.

Spider gears are essentially just like normal diffs except they have 8 gears instead of 6 gears. They offer more traction.