The GM 6.6 liter engine is a V8 that was introduced in 1996. It was produced until 2001, when it was replaced by the GM 8.1 liter engine. The 6.6 liter engine is still used in some GM vehicles, such as the Chevrolet Silverado and GMC Sierra trucks. The 6.6 liter engine requires premium fuel, but can also run on regular unleaded gasoline. The recommended octane rating for the 6.6 liter engine is 91 octane or higher.