1868 9/6 Shield Nickel?

Hello,

This coin has a few areas of discussion to cover.

1) Firstly, does the third digit of the date appear to have a 9 or another number stamped over the 6?

2) If so, is this a valuable error or just a common occurrence?

3) Would soaking this coin in olive oil for six months help to remove the black gunk that appears on this coin?

4) If olive oil is not the best choice to improve this coin's appearance, what chemical would work better to safely remove this residue?

5) What would the grade be before and after using the olive oil or another chemical?

Thanks