Use the oil, as suggested and give it a night to soak in as much as it can.
Do you have a bench with a wood top? like a work bench, reloading bench, etc?
I turned my (barrel/guide/spring removed) slide upside down, placed the back of the sight against the edge/corner of the table and give the slide a good smack with that rubber/polymer faced hammer.
Seems to me that way, the impact is on the slide, not the sight and the wood will not harm the finish of the sight (corners) and absorb a little of the impact/force to cushion the sight insert (tritium vial).
The sight should just pop right out since its easy to put some force on the slide vs. hitting that small sight, or putting a punch edge on it (small surface area).