Must we use 91 octane gas in 2018 3.6 liter engine

SOURCE: Engine Check Warning

In this vehicle the CEL will illuminated if it sees a problem with the emission system,transmission,or igntion. It will remain lit as long as the problem is present. It will go out after 2 drive cycles and all monitors complete if the problem is no longer present.
Now a drive cycle is from cold engine to warm engine then back to cold engine. The monitors are like for example your anit-virus program on your home PC. There are several of these programs on this vehicle for example,Evap(evaporative emissions), O2 monitor, Fuel system monitor, etc. For each of these to run there has to be certain requirements met like driven at a certain speed for so long,a few stop and go's, decel form hwy speed, etc. so it could take several days for these monitors to complete.
If these monitors complete and it has gone to 2 drive cycles with out a problem then the PCM will turn off the light and store a trouble code in memory.
Now can you drive it like this,the answer is yes as long as the light is not flashing,however if problem is in transmission or ignition you may cause more damage if you continue to drive it. While the light is on the PCM detects a problem and if it is in the emission system it will substitute a signal for what ever problem it see's. It will run ok like this but you may see a drop in performance and a decrease in fuel mileage.If your state has an inspection program your vehicle will not pass as long as this problem is present.
There should be a code stored in memory and there are places that will retieve the codes for free like Autozone. However they just retrieve the codes this is be no means a diagnosis of the problem,however it may help determine if there is a problem that you should have addresses right away.

SOURCE: getting better gas milage....

Hey there, well if its anything like mine(91 cressy) its pretty thirsty! but things worth checking(if u havnt already) are the engine timing, fuel filter, air filter, as they will make a huge differnce. still im getting 11L per 100 around town, and i dont exactly flog it. and about the octane, i realy wouldnt worry too much if its just the non turbo version, although sticking to 91octane or above is always a good idea. usually only costs a few extra dollars a tank for 98 octane here in Australia, so i think its worth it.

Cheers
Andrew from Queensland

SOURCE: I have a Ford Ranger that when I use low octane gas it rattles

spark knock. a second flame front hitting flame front from plug firing.

Could be from carbon build up in cylinders. clean wit chevron techron additive in bottle in fuel tank.

or, spark timing too far advanced, not likely wit computer unless someone moved distributer

SOURCE: My 1995 Lincoln Mark viii takes fuel of 91 octane

The higher the octane the cleaner the engine will stay.

SOURCE: 2008 mitsubishi eclipse syder. 16000 miles. Check

Let somebody scan it for you and see what the code is.You still have warranty if it's something major.