Possibly daft question, but how much oil do you need on the oil stone? A good covering along the whole stone? I've got a cheap £5 combination oilstone and I'm just using 3 in 1 oil on it. I assume that'll do the job?
That Rutlands one looks like it's worth buying as it's 3" and my current one is 2" which is only just wide enough for my plane iron. I saw Paul Sellers had a set of three all sunk into a piece of wood, which looked handy. Flipping them over all the time did seem like a pain.
Why are the diamond stones considered naff by some? If you're an amateur like me, only doing an evenings work once or twice a week with the odd weekend thrown in, surely they'll last me ages (if I do get some).