Usually, I would re-condition the rosewood fretboard of a new guitar on its first re-string. This is what I have been using- wood conditioner from the hardware store & it's not a guitar product. Usually, guitar-related manufacturers would alleviate product desirability by adding scent to the formula & this would more often than not increase the acidity to the content (eg: lemon or other citrus ingredients). Acidic fluid, when applied frequently to fretboards, would soften the wood & it becomes less resistant to scratches or dents. It might eventually weaken the neck strength altogether.
I'm easily bothered by nut choke (& how it affects tuning) so I make it a point to address this by lubricating the slots. I'm using commercial vaseline (bought this one at Mustaffa Center) & again, it's not a guitar related product. Use a pointed implement to apply the vaseline into the slots, in this case, a satay stick does the job.
No vaseline in the house? Vicks Vapo Rub works as well.
If you are particular about what goes into those slots & would want a safe bet, look for slot lubricants in the stores such as this GHS product. The reason why I stopped using them- they cost more & don't necessarily work better. Some guitar techs would tell you just rub pencil lead into the slots, that's the least costly way to do it but that stains the lighter coloured nut & might prove to be a point of contention during re-sale situations.
