Finishes and plastics, particularly on old guitars, dry out / evaporate over time. It is remarkable how much and well those evaporated oils can be rejuvenated with the CORRECT type of oil(s) applied CORRECTLY; Which is the idea behind high quality guitar polishes, ....although there have been some terrible guitar polishes over the years, particularly during the 1970's and 1980's, and particularly silicone based oils and polishes. I have found and proven over 20 years time now that Ace Hardware Lub-E aerosol light oil, shown in the photo above, is EXCELLENT light oil and polish for guitars IF USED PROPERLY as said here. I use it exclusively to replenish finish and plastics oils and to help fend off corrosion.
INSTRUCTIONS: Never ever EVER apply liquid oil directly onto to a guitars finish or plastics, but, using very light oils proven to be innocuous to plastics and finishes, mist the oil onto a rag and use the rag to apply oils, wiped as dry as possible with a fresh clean soft rag. NEVER ever EVER allow wet oil to run under or down metal parts as it will likely get onto wood screws and follow the screw into a guitar's wood; It doesn't take much oil down in those screw holes to turn the wood to mush.
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I apply lemon oil to bare woods in a very similar manner. Regular fingerboard maintenance only requires a drop or two on a rag to treat the whole fingerboard; But if the fingerboard hasn't been maintained and is dry on it's surface, then more lemon oil might be needed, adding a couple drops to the oiling rag and repeating the oiling & wiping until the surface of the fretboard turns the same darker color all over; And fretboards are a prime example of wanting NO wet oil to get under a fret(s) and thus down into the fret slot; So the oil should never be wet on the fingerboard, but just films applied with the applicator rag and then wiped as dry as possible while oil applied with the rag will indeed soak in during the wipe-on / wipe-off process:
