What will happen to the efficiency of a bike after the gears have been cleaned and the chain has been oiled?

Well, figures for efficiency vary a lot, but according to Bicycling Science it's lubrication that matters most - lubing a dry chain can add 5% to the efficiency.

More interestingly (and I hadn't read this bit in the book before) it varys a lot depending on gear ratio - bottom gear (22-28) is 99% efficient, top gear (42-11) is 88%. That's a big difference