Guys, it doesn't (or shouldn't) matter that you're at altitude because your mass air sensor is going to inform the engine of how much air (and oxygen) is going by through the intake.
That's the purpose of the MAS, to tell the engine how much air is being sucked in and be able to accurately measure that based on varying conditions (from pressure, to humidity, temp, etc).
So no, just because you're at altitude doesn't necessarily equate that your engine is running FAT at altitude because the sensor SHOULD be able to accommodate. Just like if you run a vehicle at 40F with 10% relative humidity vs. 100F and 90% humidity. The air content swings are just as drastic as if you went from the ocean to the mountains.
You could present an argument that the sensor is more or less accurate in certain conditions, which might have an effect on the A/F or the amount of fuel the system allows into the chambers.
I will concede that the one thing that altitude does is have an artificial reduction in compression. 11:1 compression is the same at altitude as sea level just because the engine dimensions don't change.
This WOULD lead to slightly safer conditions running crappy fuel at higher altitudes.
However, as I pointed out, with modern knock sensing and speed density, IN THEORY the engine should adjust and ensure that if you run garbage fuel, it SHOULD adjust the timing to ensure you don't get knock.
You can all do what you want. Like I said, for just putting around, I would think it's probably fine. If I was towing I wouldn't do it and if I did run 85, I'd baby the engine and not ask much of it by way of hard acceleration uphill in high rpms, etc.
I live in FL and don't even remember the last time I saw 85 available. Because again, no one would buy it, because it would fry most engines/rotating components. In 90F heat with 90% humidity and 85 octane it might as well be a compression ignition engine.
It's not uncommon for me to see 110F intake air temps (when the vehicle gets really heat soaked).
I have an NGuage on my Ford that allows me to monitor nearly 100 different signal outputs, and I usually have A/F, spark advance, Intake Air Temps and Knock (how many degrees it's pulling out because of knock).
What's particularly useful about being able to see all that is things like seeing the spark advance at cold start, seeing how much advance it adds during certain conditions (like 6th gear on the interstate) and how much knock retard it pulls out in certain conditions (If I stomp the gas in 5th at 2000 rpm, the motor will pull out sometimes upward of 6 degrees of timing because it's bouncing off the knock sensor, and this is with 93 Octane).
Even though the engine is supposed to be able to do all these wonderful things, there are also OTHER factors that come into play that the engine CAN'T anticipate or handle.
Take for instance, it only takes one hiccup of the injector to create a massively LEAN and dangerous condition on a cylinder. That's easily created if you're in a vehicle where the fuel level is low and you accelerate hard (or even climb a parking garage) and the fuel sump gets starved of fuel (and consequently the basket/pump, the fuel line, and then the injector).
I've literally seen data logs of guys that spike their A/F ratio because they romp the car with a 1/8th of a tank of fuel and the fuel system sputters because the basket isn't submerged.
This applies in a 4R as well when you're offroading with the fuel bouncing around in the tank. One bad moment and 85 is going to nuke your engine.
It's just not worth it to me. But "knock" yourself out. No pun intended.
