I do this allot- and whether you choose to use a water based or oil based system your best to clean and abrade the surface then apply an adhesion promoting primer before applying your normal paint.
some of the best primers are:
Dulux trade ultra grip
each primer has different for's and against depending on the situation depends on which one I’d choose
Oil paints should go on to oil no problem. You can use raw linseed oil direct as a primer under oil paints, and they stick like the proverbial pineapple to a blanket.
You might need another sort of primer for non oil paints.
Thanks both. I can't abrade the wood before hand as its very small engraved lettering. So your saying I should hit it with a bit of primer and then I can use a normal paint? Or am I ok to just get in there with the oil paint straight off the bat?