What gas does Costco sell?

In most areas it will be the cheapest local distributor, ie: generic gas.

 

I think each Costco buys from local distributors, the same for all other brands. The difference is the type and amount of additives each brand uses. I can't remember where I saw a write up about their gas, but in it Costco stated they use 2x the amount required for additives in their gas.

 

I have heard that with everyone running E10 it naturally has all the cleaning agents your motor and injectors need.

 

I believe that is incorrect. In particular the auto manufacturers feel a fairly high dose of detergent additive is needed to prevent gunk build-up on fuel injectors, etc. EPA requires a lesser dose than the auto manufacturers like.

Some other additives are certainly impacted by ethanol, for example de-icing additives to prevent fuel line freeze ups would probably not be needed as much anymore.

 

My MPG is down with Costco or Sams gas compared to name brands

 

Mine is consistently better with Costco gas.