Can I fill my gas tank with E30? I notice others with my car doing it but I want to make sure before I do that.
If your car is designated as a "flex-fuel" vehicle, it will be just fine. However, if it wasn't, don't run anything greater than E10 fuel (10% ethanol). E30 is 30% ethanol by volume, which will degrade sealing components and soft parts in the fuel system if the vehicle was not designed to take it. Ethanol is caustic to these parts by nature. While you won't see issues right off the bat, it will degrade the system and start causing issues later down the road.
Bottom line, if your car isn't made for it, don't use it. Some vehicles have multiple designs where the same model can be built either way. Know your vehicle is flex-fuel capable, then you will have no issues.
People do that in Brazil all the time
Granted, most cars there are already designed to take E30 and usually imports have to be modified, but many imports don't have any modification at all.
Yet take this with a grain of salt...
While filling a gas car with E30 (and sometimes even with E90) is likely to do little damage, be aware that you can lose warranty and some components may get worn faster due to the higher temperature and lower mass-to-energy ratio of ethanol (which leads to higher strain in the fuel pump and fuel injectors), specially if you make this a habit. Any rubber part in contact with fuel also will corrode much faster. The car also runs rougher when cold and may be more difficult to start in cold weather. Carburated cars will suffer more roughness than fuel injected ones.
TLDR: Usually works, but try at your own risk
PS: Mention your car year/make/model/motorization/market to get a more precise answer. This answer is highly car and country dependent.
Car now days are made to be more ethanol tolerant since gasoline with ethanol is common. If you look for parts for a flex fuel vehicle and its regular version you will find the parts listed identical. The difference being the computer programming. 1996 and newer computers should adjust well to E30 and maybe higher in a regular vehicle. Unless your vehicle is older than a 96 I say try it. My 2003 runs just fine on it. And since most newer engines are higher compression the computer usually retards the timing on the cheaper gasolines, E30 is a high octane fuel, so using it is like using cheap premium and I know my vehicle seems to burn less compared to the 10% they want to limit me to.