Me, too. I’d leave it alone. Maybe a little oil on the stock, but ultimately it’s your call. Just bear in mind that if you cheap out on the refinish it will look it. I don’t know what used M61s sell for, but I can guarantee that a good refinish will cost more than the rifle is worth, in all probability.
Just shoot it like it is, remember those who came before you, and don’t worry about what it will sell for after you’re gone. It won’t matter to you then, anyway.