Loosen up C25 faucet handle?

[LuckyDuck]

The handle on my galley faucet is really stiff — almost impossible to move forward and backward. I’m afraid to force lest I break it. I assume there’s an O ring of some sort inside that needs lubrication. It appears the black retaining ring has flat sides and can be unscrewed — is that the solution? (Picture attached.) Any suggestions are very welcome. Thanks!

 

[sail sfbay]

If the nut is metal you could heat it up. If not, apply a penetrating lubricant such as Liquid Wrench or WD-40 to the fixture or nut and allow it to soak in for at least two hours. If you see visible mineral deposits, apply a calcium, lime an/ord rust remover to the fixture and allow it to work. Scrub off the deposits with a wire brush.