Actually it means that half the recoverable oil will have been used up but half will be left. (NB "oil" is uncountable so you can't say "these", you'd have to say "this".)
As for the train sentences, they both mean the same thing but the first one sounds better. Maybe with some verbs (other than "leave") the choice would make a difference but I can't think of one offhand. Maybe you can?
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