What does "take a toll" mean? Sorry to be difficult but people tell me a lot of things will damage my car in the long term, but I haven't been able to reproduce any of those problems yet. The only thing that damaged my car was putting too much oil in it, that caused it to fail nearly immediately, and was covered by the warranty so it was free to replace the whole engine.
Eh, your car will run a little shitty with the old gas, leaving an engine without running it for extended periods can cause somewhat faster corrosion depending on your environment, and old gas can increase things like various gunk getting deposited around your engine.
All in all, a mild shortening of the life of your engine and perhaps triggering some maintenance sooner than otherwise.
Realistically, not much. The people saying “bad for you” aren’t wrong but they also fail to mention the effect size. It’s not particularly relevant unless you want to own the same car for a few hundred thousand miles and absolutely minimize maintenance. (Small aircraft for example, you want to care a lot about these things because they’re very expensive, when the engine fails you’ll be thousands of feet above the ground, and a well maintained plane can last decades. Your daily driver probably doesn’t have any of these issues)
There are various preventative things you can do to minimize the effects anyway. It’s one of those internet things where yeah they’re not wrong exactly but they need to relax. Having too much beer and cheeseburgers last night long term probably wasn’t the best decision, but you know what I’ll probably be fine.
Leaking gas in a car for a long time is like eating too much fast food. Ok yeah not the greatest but are you the type of person who cares enough to never eat McDonald’s? Make your decisions according to your disposition and don’t take people telling you you’re wrong too seriously.
But seriously, change your oil more often. There’s some simple chemistry going on where the oil gets worse at being oil over time and you get extra engine wear, and if you’re not paying attention and burning oil your engine can get bricked in seconds.
I only change my oil every 10k miles (or really 6-8k most often) but in an engine and with an oil specifically rated for such. Your jiffylube 5w30 (if that’s what you’re doing, if you’ve actually got an oil rated for 10k you’re fine) isn’t going to do great. It’s maybe 50-50 whether bad oil change schedule will be the thing that kills your car.
I think 6K or 7.5K miles is the typical recommendation for modern cars that use synthetic oil. I know my BRZ's manual recommended 7.5K unless I'm taking it to the race track.
10K is probably fine with full synthetic oil if you're driving conservatively.
Ethanol is hygroscopic. As long as you burn the fuel in a reasonable amount of time it's not a problem, but when ethanol sits over time it collects water. Next time you run your engine, it pulls water into the engine.
Now, if the engine runs long enough to fully warm up, this isn't the end of the world. Any water will turn to steam and be pushed out through the exhaust. However, if the engine only runs for a short period of time, water can sit inside of it and cause corrosion. Realistically this is only a problem if you repeatedly allow ethanol-containing fuel to sit for a long time, and also repeatedly run an ICE for very short periods of time.
It is a bigger problem for boat motors, because they operate in a wet environment already. It is also a bigger problem for carbureted engines, where fuel sits in the carburetor bowl while the engine is not running. This allows hygroscopic fuels to corrode the inside of the carburetor while the engine sits.
Ignoring ethanol, it is also true that gasoline breaks down over time. However, it typically takes many months before gasoline breaks down enough to worry. There are also additives that help stabilize gasoline for longer storage.
TL;DR in a reasonably modern-ish car, it's probably fine. In small engines like lawn mowers, it is best to either use ethanol-free gasoline or to completely drain the fuel before storing the mower for the winter. In antique cars with carburetors, it is best to use ethanol-free gasoline.
