I think its Mid-Grade.
The minimum octane rating is on the gas cap door.
Follow what GM recommends.
While the engine's knock sensor will detect when you use a lower octane fuel than what the engine is designed for and retard the spark plug firing to compensate, it's not a good idea. This is because it immediately does the following:
-lowers engine output and
-decreases fuel economy ( thereby erasing some of those saved pennies ).
In the long term, continued use of the incorrect octane fuel can actually harm the engine.
