I doubt your tubes in the sills will leak - they will probably leak at the connection points if anything. But yes - there are lots of oil and coolant lines running back and forth through the sills. That's one of the reasons the cars can be hot!
I guess the car will hold a lot of oil (I guess it could hold a few quarts of oil in all that tubing and radiators). That will be good for a hig revving hard working engine. The down side is when you change the oil you will be leaving a good amount of oil in the system.