Hi, not a car expert here, but I do have 2019 Forester, and from time to time I leave it in my garage for up to a month either for vacation or helping my parents to burn some miles on their low mileage cars.. I don't think it was big deal. The car runs fine.
+1 on point that you needn't think about fuel stabilizer for under 6 months. And the whole water condensation from the air in the tank? To the extent it could be a theoretical problem, it would happen as a function of temperature, not time. Water in gas issues have all but disappeared in the last 20 years, for a number of reasons. Especially if you do not pay extra to avoid ethanol in your gas.
Cars, on the other hand, have been loaded with electronics that use the battery 24x7. Designs call for the power draw to drop off after an hour or two of stopping, but sometime things don't work as designed. Many vehicles of recent vintage will run down a good condition, fully charged battery in a few weeks.
Since the problem tends to be with draw, two different approaches can work. Replace the parasitic draw with a battery maintainer (trickle charger) or prevent it by removing the neg cable. Depending on the power distribution design, removing one or two master fuses might accomplish the same as disconnecting the cable. Your choice, either works well for a month or two or even three.
I've left my '02 minivan with a 5 and then 6 year old battery, sit idle for Jan and Feb, and have had it start right up, with no precautions. But not a lot of things drawing power continuously. People with recent models find dead 6 month old batteries after 2 weeks.
To the electric part, buy a Ctek MXS 5.0 charger.
Put it on recondition the first time U use it.
If your car is out of use for a month it change automatic for service charging so no problem for your battery.
Then if put on winter program for charge every 3 month it does the same and make your battery life last extra.
If U do your self or get a garage to fix a quick connection that comes with it will make it easier for use.
It will be one of the things making 2021 a bit better and lovelier 🤠🤠
We have left our Subaru Forester Limited several times parked in our driveway and in our daughters in Boston for trips of one month to Oregon (twice) and 3 months to Spain (2 times) in September, March and May. We left the car with no trickle charge and no fuel stabilizer
When we came back I was agonizing about starting it...but start it did at the first try all the times.
I put the key in the ignition and turned it to just before starting it til I saw all the lights and gauges on...them started !
You don't need to do anything. No stabilizer, no battery charger. I've left our 2018 many times for 4-5 weeks and it starts right up. I'd also be uncomfortable using any sort of fuel stabilizer...that stuff is for lawn mowers, generators, not sophisticated automotive fuel systems. A trickle charger/battery minder could be a useful tool for longer periods of non-use, but a new battery and for only 4 weeks that should not be an issue. Just make sure no lights are left on.
We leave our '17 Forester XT in a garage from October to May (8 months) at times. Use a trickle charger, full tank with stabilizer, and flat stoppers for the tires. No problems.
We once omitted the trickle charger (I was concerned with leaving it plugged in, unattended) and, of course, the battery was dead. However, AAA restarted it and it's been fine since then.
Mercedes Benz is a different animal. We left it alone and expected to return in 2 months; however, COVID delayed our return for a month. After 6 weeks I started getting text messages the battery needed attention, but there was no way to charge it because we were gone. Eventually the messages stopped...when we returned, I charged the battery and it started OK. HOWEVER, when I took it for service, it failed all sorts of electrical tests and they replaced the battery and other items (fortunately under warranty). The owner's manual does warn not to leave it uncharged more than 6 weeks or bad things will happen...they were right!
LONG-TRIP : For the Duration that you mention :
(1) full fuel tank w/o additives; (2) battery "maintainer" best answer (they are slightly different from a "charger", do a GOOGLE Search); and now for the "secret" that SUbaru does NOT want owners to know about .......
(3) if ever stuck w/o access to AC-Power (airports, etc.) there is the SECRET option ... of pulling the "BackUp-Fuse , which is NOT for the Back-Up-Lights : it is the "mystery" fuse that is pulled at the Factory after final assembly & System-Testing, before it is shipped to dealers; it kills power to all the Electronic Systems that slowly ruin the battery if the Vehicle sits on Dealer-Lots for several months.... If you do pull it, you may need to re-set some of your Electronics when you return to your Vehicle ...... : it is one of the Listed-Fuses under the Hood, usually a small fuse fairly near the Fuse-Puller Tool storage , check you Cover for its Name, and Fuse Number. It sits there in full view, awaiting "discovery" !
I would like to relate a storage story which while it does not involve a Subaru is a cautionary tale. In 1973 I parked my '69 GTO convertible on the 2nd floor of a storage garage while I attended the S. E. Asian war games. The car was to sit for about 14 months. I removed the battery, drained the gas, ran the engine dry, used stands to take the weight off the tires (Kelly Springfield Wide ovals), and covered the car with a breathable cover. All was good, except the garage caught fire about 10 mos. into the storage and my GTO along with about 50 other vehicle were lost.
So, you never know....
Optimate and C-Tek make decent battery conditioners/trickle chargers that are designed to be left on the car for days, week or even months, where it will keep the battery in a condition you can get in and start the engine straight away. No need to disconnect anything with the charger on too. I've had mine for years and very happy with it. As you have start-stop, you might have an AGM battery? So check whichever charger you go with, can cater to your battery type.
If you're concerned over fuel, run it very low, then fill up with ethanol-free fuel. Sunoco used to sell it but unsure personally who else might sell it these days.