OIL started out 20 years ago as an entry for one of those 24 Hours of ZZT contests. The contest entry, while quite short, had a very pleasant art style, setting, and atmosphere, but was unfortunately marred by a very poor action sequence. Still, in spite of the (perhaps unfairly) low rating I gave it (when the judging for the contest finally happened in 2018), it was easily the entry that I most wanted to see more of.
In short: boy howdy is this a good ZZT game.
In making this full version, dave2 wisely chose to elaborate on the most compelling aspects of the prototype --- the underwater setting and atmosphere. The flora and fauna of the place are both novel (for a ZZT game) and distinctly aquatic in flavor. The colors of the coral reef, natural rocks, and deep water all work together in a pleasing way, and provide a nice contrast to the man-made oily blacks and steely grays of the crashed tanker. The denizens of your mermaid colony also bring life to the setting, having distinct personalities and functions in the game.
As an adventure game, it's quite solid. While the game puts you on a global timer to stop the disaster (which is disabled on easy mode), it tends to be quite generous, and completing quests and other tasks on behalf of your fellow merfolk increases the amount of time you have remaining. Navigating and exploring around and within the wrecked ship is interesting, with the routing options and puzzles expanding in lockstep with your understanding of what needs to be done to remedy the disaster.
To top of all off, the action segment from the contest version does make a return somewhere during the adventure... and it's actually pretty decent.
There's the occasional rough edge here and there, but overall I'm willing to call this one a Certifiable Modern Classic.
(The only bug of note that I encountered is that I somehow locked myself out of collecting the trident. Fortunately, it's not necessary to complete the game, even if it would have made things a bit easier by the end.)