I used tung oil on the jarrah, maybe 6-8 coats. Very dense wood. Given how hard jarrah is I'm not sure it would translate to a warmer tone. I would imagine your choice of body timber and pickups would have more of an impact.
I've used Jatoba for several necks, though never a fretboard. It has a similar look as Jarrah, and is extremely hard and dense as well. It has a reputation for adding midtone warmth, and my guitars with jatoba necks certain have plenty of that. How much the Jatoba contibuted to that will always be debatable....but it's food for thought.
SR
Thanks guys, I had read that Jarrah has good warmth and mid tones, but I was unsure as they're characteristics not usually associated with hard, dense woods. Like you say curtisa, with it only being the fretboard it probably won't have much impact overall. I've ordered one now and when I eventually get my build back on track I reckon I'll start a thread so you guys can help me on my way!