I have been following these thread's for quite a while now,since I found this site.I have noticed there is some people out there that may have some knowledge,experience,and perhaps some professional root's in the field of gasoline consistency.
I know,by paying attention,that gasoline seems to vary in TRUE Octane rating across the country (why I don't know) My question is...should I top off my gas tank and keep it full at all times in order to offset for the additives I put in the tank like...MMO And gas stabilizer,that I use over the winter months, when my car sleeps in the garage for it's winters nap....How does this stuff(additives) affect the octane in the gas and the requirements of my 216 to perform at it's best, and at the same time maintain it's health ????...Thanx Ed

Ed
None of my vehicles get a chance to "sleep" but if they did I would put a sizable amount of your MMO in the last near full tank of gas then use most of it prior to resting. This will coat the inside of the tank therefore providing some rust preventative there and also lube all of the fuel system.
In any case I would not leave a large amount of fuel in the tank for long periods, to reduce the cost of evaporation and prevent large quantities of stale fuel to cope with at next start up.
Tony

Keeping the tank full of gasoline was once the recommended long term storage condition. That was because the temperature changes during storage caused the tank to take in and expel air that contained moisture. The moisture would condense on the exposed metal and then drop to the bottom of the tank eventually causing rusting of the bottom and exposed upper parts. If the tank was full then only a minimal amount of water would enter the tank.

Today there is another problem. It is oxidation of the unsaturates in the gasoline. An anti-oxidant is added to gasoline and protects for ~90 days of expected storage time. In addition to time and temperature (higher temps accelerate the oxidation) the amount of surface exposed to air is a factor. So the lower the gas level in the tank the more air to gasoline ratio and the lower the amount of total antioxidant present. So the lower the level the faster the gas will oxidize and form gum and varnish.

The solution? Add additional antioxidant and keep the tank full or drain the tank completely. If the tank is completely drained then internal rusting can still occur. But if the tank is full and the gas oxidizes then you have a bunch more rotten fuel. The decision is yours.

Or remove the gas cap and seal the opening with tape. This will eleminate evaporation and prevent moisture from entering.

The tape can reduce the venting of the tank but not likely eliminate it particularly if the tank is empty or mostly empty. The change in temperature causes the gas (both gasoline vapor and air) in the tank to expand and contract. If the tank is totally sealed then the tank will flex under the pressure. If cold enough the tank will suck in. Not a good thing.

A latex or nitrile glove over the filler neck would provide a sealed environment but still allow room for expansion/contraction...