The game is autosaved each time you start a new level, the autosave overwrites the older one. Then at the end of each world, you can save to a named game save.
You've got to learn to keep your lantern burning low until you need it. Once you master the lantern, oil is never a problem.
If you want to extend the playtime of the demo, just pop into /Media/Config/levels.cfg and turn up all the map sizes. You could also try making some new types of shroom and weaponry, it's all quite easy. There are some tutorials starting to appear on our website, at
www.chronicreality.co.uk