Loosen the strings so you can slip them to the side of the nut. Oil the fretboard. reposition and retune strings. No big deal.
There's a couple of rabbit holes hereabout that are easy to disappear down. I read all manner of stuff in books and on the 'Net. Yes, lemon oil is just lemon scented mineral oil.
Expensive as Hell for what it is. Then I saw the Martin YouTube clip showing the factory using 3in1 oil on new guitars. After that I'd have been happy using 3in1 but I'd bought some nice orange scented stuff meantime.
I've read warnings about taking all the strings off, but doing that makes cleaning the fretboard and frets easier. So far so good.
I really like LoPrinzi fretboard conditioner.