General question. As far as oil consumption rate, will an engine filled with the proper amount of oil burn oil more rapidly, or will an engine that is low on oil oil more rapidly? or will it make no difference? Also, I'm guessing old broken down oil will burn more rapidly than fresh oil?
Oil consumption shouldn't matter whether in a high or low state. The engine will use it at the same rate. There is nothing there telling an engine: You're low on oil! Burn, baby, Burn! I would say, though, if an engine is run extremely low on oil, you'd see the rest of the oil in it come out faster. This would be due to an extreme amount of wear going on. Once this happens, you'd start seeing a regular increase in oil consumption, though, whether new or old oil.
You are correct older oil will be consumed faster than new oil. This is due to contamination in the oil, mainly from gas (fuel) which will thin the oil, making it easier to escape, whether out the tailpipe, through leaks, or back in through the breather (which also equates to going out the tailpipe). Usually, the small amounts of oil which are consumed, either through loss or through burning, are usually made up for in fuel. Most of the time you'd never notice the loss of oil between changes due to this.
Every car consumes some amount of oil. If the mechanicals are in good order, that amount is usually pretty low. Some vehicles are just prone to consuming oil. Take for instance the LS engine series from GM. When they first came out in the late 90's, it was quickly revealed most of the consumed oil faster than what most of us would call normal, however, GM considered it within spec, so not much was done about it. They did change something after a few years which helped with the problem, so that was good. Point being, it was pretty natural to have an LS based engine which ate oil. It happened; you got used to it. Other engines aren't any different.