I've oiled my vintage Hagstrom guitars that have real rosewood fretboards. The one I've had since '75, maybe a handful of times since...the others 1-2 times because I've not had them as long, and only when doing full setups if they looked dry.
Many of my newer guitars, which are Hagstrom reissues...all come with their "Resinator" fretboard, which is resin impregnated wood...so it's pointless to try and oil it, as it will not absorb anything at all. They other newer guitars, I have not bothered since some have maple necks, and the rest don't look like they are in need of any.
Oh...in case anyone doesn't know..."lemon oil" is really a silicon based product with just some lemon scent in it. There is no real oil of lemon in it at all.
It's good at keeping moisture and hand sweat from getting into the wood...but not sure how good it is for the wood. There's probably better stuff that will "feed" the wood more naturally than silicone...but I don't think it's all that critical unless you live in a very dry climate where the wood can dry out substantially.
Look at the fretboard...if you see any fine cracks in it, then maybe it is drying out too much.
I think simply keeping your guitars in a favorable environment with the right amount of humidity is all that is needed AFA any "treatment"...other than regularly wiping them down after use, and occasionally using a basic guitar polish/cleaner...like the Dunlop stuff.