Completely unscientific but here it is for what it`s worth,

I took 25% ethanol gasoline out of the boat 10 months ago and treated it with amsoil and Stabil 360.

(2) Stabil360 bottles , one open to atmosphere, one closed
(2) Amsoil fuel stabilizer (dark in color) one bottle open , one closed.
(1) 25% ethanol left open (non treated)
(1) 25% ethanol bottle closed to atmosphere. (non treated)

After 10 months the open 25% non treated get cloudy when stirred and I can maybe see some seperation... doesn`t seem to burn as well as the others.
The rest open or closed I do not see any difference since 10 months ago .

Video of both untreated, one closed (still looks fine) the other open (cloudy)

Untreated bottle of 20% gas 3rd row left.
The rest that have less gas are all treated but left open. (some gas evaporated)
The ones that are fuller were closed.
Front row right side is untreated but closed to atmosphere.

Untreated gas open vs closed

Untreated and left open (after stirring) on the left .
Treated and open on the right also strirred.

My conclusion:
TREATED ethanol gas is fine ....
Closed untreated ethanol Gas is also fine.
Closed gas tank and check valve vent should keep untreated 25% ethanol gas fine for a while.
Fuel stabilizers work.
I cannot see the dreaded "phase seperation" in the untreated gas. I would have thought it would be floating on top or bottom like oil and water ...
10% ethanol would fair even better untreated.

Wish I could test the octane.


Next test is how long will it take for the untreated gas to turn cloudy and/or show seperation.

This is what I was expecting :

interesting stuff. Someone on here also did a pretty good test involving octane ratings that delt with fuel age as well as advertised ratings. I think i read somewhere that stabilizers essecintially float to the surface in the tank basically sealing off the fuel to open air and that would support your findings as well.

Great thread Dan. I have some treated 10% ethanol treated from 2020 in a 100gal portable tank I'm going to have a look at this Spring to see how it fared.

The octane rating issue begins at the pump. Who knows what the actual octane rating is these days despite the advertised Ron/Mon.

There was a big test here in Vancouver area on octane. The high end premium fuels often didn't match up to the stated octane. Started from a factory perf car with required high octane...engine failure under warranty. Manuf said it was poor and low oct fuel. Customer had documented every fuel fill of 94 Oct. The focus became the fuel stations...and the trigger for the local perf community testing.

This is 30yr old gas from1993 alum boat fuel tank. Sat of 30 yrs. Pretty much varnish now. I'm amazed that is still smells like fresh gas. No skunky old gas smell.

It was vented as per normal.

I was expecting some nasty tanks inside. Was blown away with the pristine condition of the tank.

In Illinois the state tests gas stations to make the octane rating is as advertised.