About how little oil can I add to an amount of flour to make a dough that is one lump of one consistency, not crumbly and in pieces? I'm looking for a ratio of solid to liquid. If it matters what kinds of flour and oil, let's say olive oil and white wheat all-purpose flour.
This is actually not answerable as an absolute. One of the Great Unknowns when baking with flour is how humid it has been in your flour storage recently, and thus how much water is already in the flour. I've had a few cups of flour require as much as half a cup extra water to get the right consistency depending on the season.
My usual approach for making the driest dough I can is to instead of pre-measuring, add the liquid gradually to a spinning food processor that contains the dry ingredients. It will lump up quite quickly when the liquid is right.
According to the experiments reported in this paper they found:
We determined by experimental evidence the maximum amount of olive oil that could be absorbed by modern wheat, with a minimum of water to make a viable bread prepared as described. It was 5 parts oil to 16 parts flour, to 4 parts water.