Several car manufacturers recommend using gasoline which meets additive standards called "Top Tier". The reason for this is that many gasolines on the market do not contain enough detergent additives to keep your engine clean and running well. One of the byproducts of incompletely burned gas is carbon which can coat the inside of your engine's intake system, valves and injectors.

The detergents in good quality gasoline help to clean these deposits off. If it is not cleaned off this carbon can absorb the gas as it is injected into the engine and create a poor mixture of air to fuel. Carbon can also keep valves from closing properly or clog the fuel injectors.

The most common and hardest to detect is coating of intake parts resulting in a poor mixture. This can result in poor gas mileage, low or rough engine idle or intermittent stalling, but without taking the engine apart to see (and clean) these deposits it can be very difficult to know why it is happening.

Therefore we recommend using good quality gasoline (of whichever octane the manufacturer requires) in all cars. We may also recommend to you that we add extra detergent additive to your car's gas tank if we think there is a potential problem.

Prius seems to be particularly sensitive to bad gas. While there are many causes for poor gas mileage (most important being tires and tire inflation) we think gas type might be a big contributor to disappointing mileage in these cars. It also seems to be implicated in several cases of intermittent engine misfires. In serious cases codes are set in the computer and the check engine light can come on.

Most of the major oil company gasoline meets "Top Tier" standards along with some obscure brands, but some gasolines that are in common use in this area are not on that list. We would recommend you avoid these. The list of brands that qualify can be found at TOP TIER Detergent Gasoline Retailers on the Internet.