Seriously, I haven't oiled a fingerboard in 20 years. I stopped when someone told me I was oiley enough myself. Once or twice a year I'll degrunge a set of strings with some Fret Ease (or whatever it's called) but I'm careful not to get much directly on the fingerboard. I figure any kind of oil promotes the collection of grunge, and I'm sure I'm dirty enough. Even less frequently I clean the 'boards.
Mooh's 2 cents.