Egypt 5 Mills 1938 Error Coin Clean Or Not

Egypt error coin, my question is, this is obviously a error coin but is very grimmy should I clean or not, the detail on the sections of the coin that don't have errors are fine, if I cleaned it perhaps in olive oil would this not help in the value of the coin. I know the question is going to be how do I know its a error coin, well the coin was struck after the gouging of the metal because the coin was struck into the hole in the blank (date visible)and the symbol that looks like a S clearly shows how the point folds into the hole. And does any one know the value of this coin as a error coin. It is a 5 milliemes 1938