A quick google reveals quite a few people using boiled linseed oil, so I'm sure it's fine.

The fret board is just wood after all, so long as you are consistent with what you uses to maintain it I don't see any problems.

I finally got an answer from one of the guys on Facebook who was very 'all caps' adamant that lemon oil is a bad thing. He made it sound like the guitar might burst into flames from all the 'acidity'. When I pushed him for specific documented examples of what can happen he said :

"Mainly fading of color, very small dry cracks that appear over time, and the fact that the pores (at least with Rosewood) seem to open up. It generally shows a lot of signs of something like a chemical burn."

Which to me sounds like garbage and he's just one of those people who wants to seem like an expert. He couldn't produce anything other than anecdotal evidence either.