you havent said if youve checked your oil yet. if you are losing a quart/20 miles and cant find any external leaks (they would be obvious) its in the crank case or the combustion chamber. if it is in the cylinders it could be coming from a cracked/warped head, head gasket, intake manifold.

this might be tricky! if its in the cylinders it wouldnt leave a trace, it would leave as steam in the exhaust. but if it is losing a quart every 20 miles that would be roughly equivalent to running 25% water in your gas tank! i think it would run terrible.