Hi,
Since posting this the other day, I've been keeping up by reading the responses that come in, then researching the answers.
One must bear in mind that none of us are experts in every area, and even experts will not subject everything in their field to complete scrutiny. It might just be that they 'like' a thing - like lemon oil. So why investigate something you're happy with. But the Luthier's creation can outlast the luthier. So, the luthier might not see the long term effects of a product on his work.
I used lemon oil exclusively on my Ibanez Artist EQ 'Steve Miller' model I bought new in 1977. While the fretboard and inlays seemed t do well, the bindings deteriorated (and even this I can't say that was the lemon oil's fault. However, Lemon oil is a lot more corrosive than other oils available also, it works quite well as a solvent, and there's no telling what effect it's going to have on various glues it's in contact with over a period of time. However I've no run into any links that that blamed it on their fretboard delaminating, or anything else like that.
The oils most preferred (in my unscientific search of the internet) are tung oil which is Linseed oil which is Linen oil, teac oil, Teak oil, and Crisco (which will ruin both your guitar neck and your human body.
Some boil the Linseed oil before application. People don't like it because it never really dries. People like it because it never really dries. Depends what you like.
Then there is the, What is proper for Ebony/Rosewood fretboards, isn't proper for Walnut fretboards. problem. Another problem is that some of those are varnished.
Then there are those of the persuasion that you don't need to oil your fretboard. The wood has oil in it already, so all you need to do is clean it.
Or it seems the guitar world is crying out for a custom made case with a built in sprinkler system to dose your guitar everytime you close it up.
Personally, I think I'm going to stay with a small amount of Teak oil applied 2X a year and wipe off the residue the next day. I'll fugure out what to do with my Walnut fretboards, but I think the idea here, and with necks in general is moderation is the key.
Also, Linseed oil might be good for your guitar, but it's absolutely wonderful for your body. Has lots of Omega-3's in it. Buy it in the cold-pressed version, throw spices in it, and you've got a great salad dressing, that can double as a fretboard dressing - in a pinch.
Thanks! Treader