I've never been satisfied having Oil just be a clone of Spoils, but previous attempts to make Oil something interesting didn't work, especially early on when I was stuck thinking of Oil as being like Potions. Now that I've moved away from Industry for a bit, I've come back to it with a fresh(er) mind.
My first thought was on making Oil have a variable in it, so you might not always get the same amount of money. So it might be a Treasure card, for instance, where you draw cards. Since this is happening on your Buy phase, Actions are useless, so you're only hoping for money at this point.
But thinking on that line, I realized that's much more the sort of thing that would go in Will of Ra than Industry, so if I expand Will of Ra into a moderately-sized expansion at some point, I may want to revisit that idea.
My next thought was of City from Dominion: Prosperity, which gives you bonuses based on how many supply piles are gone. I always liked that concept, but that was the only official card that was ever used on. I think, like the Clean-Up phase, cards which get more powerful late-game could be an expansion all on its own. I've teased that concept around in my head, and perhaps that will come of something, too.
But for now, I'll stick with just a demo of that with the revamped Oil Card. My first thought was that perhaps you start with +$2, but you get an additional +$1 for each empty supply pile. While it works thematically, it wasn't varied enough from City. The bonus the player should get from Oil should be something more that one of the standard pluses.
So, of course, what's important to get late game? Victory Points, of course. So with that, playing an Oil Card with at least one supply pile gone also gives you an Estate. It's a good thing, then, that Oil is a one-time-use card.
Play, test, comment, enjoy!