The secret to using penetrating oil is time.
Even a good penetrating oil sometimes just needs time. First, follow the label on your choice of oil.
A rule of thumb is a 24 hour soak, followed by a few taps with a hammer or rubber mallet to set up some vibration in the threads to distribute oil and friction.
This procedure can be duplicated as often as you feel comfortable with and it depends entirely on your time frame allowed.
It's not possible to nail down a set day(s) or hour(s) as every situation and climate is different.
Pop would, when working a bolt on his Model A, often squirt the bolt once a day for two weeks, then give it a rap to try to loosen corrosion before walking it out conventionally.
Sometimes it works, sometimes it doesn't.
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