Whilst I have limited experience with wooden screws, I did make
three 'all wooden' sash cramps last year and can relate that when I
first made them, that the wood (which was straight out of the outdoor
wood racks sitting at about 14% moisture) was very tight when I first
assembled them, and squeaked when being turned, after leaving them in
the workshop over the weekend they went down to about 12% moisture and
moved quite freely and have done so ever since. 

This leads me to
believe that the matched cutters for making the wooden screws were
perhaps calibrated to take this into account. 

But it mainly makes me
think that the only reason they should be tight is if they are too
moist. 

Also that adding oil could make them swell back up again, I
guess if you are making your own cutters you could allow for this, but
you'd have to experiment with store bought cutters. 

But to my mind an
existing or old wood screw and nut should be correct, so unless they are
coated with something jamming them up either the screw or the nut has
got moist. 

Michael