I'm afraid you are falling into an old tale. In fact, long ago the small case ventilation fans in computers were made very much like any other small DC motors, and you could PARTIALLY disassemble some of them to reach the bearings and add a small drop of oil. I've done it, and it can stretch out the life of such a fan. HOWEVER, modern fans in computers are of a very different electrical design now using "brushless" technology, which also dictates a significant change in the mechanical details. Today the moving electromagnetic fields are generated by an array of stationary coils around the outside of the moving rotor, and their fields are changed by electronic switching by a circuit board. The moving rotor carries the fixed permanent magnet mounted around the shaft. The shaft with its magnet and two bearing ends are permanently sealed in the motor bearing sleeves, with one shaft end out to the fan blade assembly. That assembly is permanently glued to the shaft end so you cannot remove and replace it, and it includes a shroud that extends back over almost all of the motor to protect it from dust. Even if you could remove the fan blades, you could not reach the bearings, and you probably would never get the blades off in one piece so you could replace it.
For a noisy fan inside a PSU the best option is replacement. Fortunately, almost all are plain 12 VDC fans VERY much like case ventilation fans, and of the same size. Your FIRST step in this would be to check the power supplied to the existing fan with a voltmeter if you can. I'm sure it is 12 VDC, but you do need to verify the polarity. Make sure you know which terminals the fan is connected to inside the PSU, and which of those is the + one. Next step is size. For case ventilation fans, the "size" in the name is the length of one SIDE of the square fan frame. Typically these are 80, 92, 120, or 140 mm. Then also check the fan thickness - most are 15mm, but check. From your post I suspect you already have the old fan removed, so you can see exactly what mounting holes are used to fasten it into the PSU.
Now, go to your computer parts shop and find a fan of that size. As to fan TYPE, almost ALL PSU internal fans are the simple 3-pin Voltage Controlled fan, so don't worry about PWM fans. On all of those, the three wires are: BLACK Ground, RED for +12 VDC supply, and YELLOW for speed signal being sent back to the mobo. I suspect that, in your PSU, the old fan did not even have a Yellow wire to anywhere, so just cut this one off short and tape its end. Mechanically mount the new fan, connect Red to the +12 VDC terminal in the PSU, and Black to the Ground terminal. Make sure there's nothing out of place like a wire where it can hit a fan blade, then close up.
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