I have never had the simple application of a penetrating oil allow me to remove a stuck bolt. Don't get me wrong, I own it and apply it, and drip it on the floor and self. It may help once you ever get the fastener to move, but I really don't think it is a magic panacea.
I have had excellent results with a hand impact. You need to use it before the slots are ruined in the head of the bolt. Here is a picture of one:
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