Gee, even the good valves had some build up after 5K. Wonder what mine look like after 50K ... lol

 

Amsoil Premium fuel additive formulated to thoroughly clean and restore peak performance in small engine and powersports equipment fuel systems.

Degraded fuel presents a major maintenance issue in small engines and powersports equipment. It can form varnish, gum and insoluble debris that clog carburetors, fuel injectors and fuel filters. Carbon buildup can form on the tops of pistons that leads to pre-ignition, rough idling and poor throttle response. Fuel-related problems are only expected to intensify in the coming years as the ethanol content in pump gasoline continues to increase.

P.i. Performance Improver Concentrate (API)
Improves fuel mileage an average of 2.3% and up to 5.7%. Reduces emissions such as hydrocarbons (HC) up to 15%, carbon monoxide (CO) up to 26% and nitrous oxides (NOx) up to 17%. Restores power and performance. Reduces the need for costly higher octane fuel. Reduces noise from carbon rap and pre-ignition. Offers better vehicle drivability and smoother operation.

 

Conoco for me at least locally. Still ethanol free here. Have used Chevron for years also.

As to Costco gas, they generally have high quality standards and from what I understand their gas is a top tier brand wearing a Costco wrapper.