the 40-s is not a key date. Over 8 million were minted. as far as toning, I'd more call it tarnish than tone. Your coin is in average circulated condition and is basically worth melt value
I has maybe a slight premium over melt, but, I agree with jewelerguy, it's not really a key. You can clean it or not, it really won't change it's value any. If it was a higher grade and had some nice rainbow toning, I'd leave it alone, but, your coin has a rather ugly appearance IMO, so, I'd clean it if it were me deciding.