Hi Mike
There are no dumb questions...sometimes we get dumb answers.
Ethanol has an high octane number. When they blend in ethanol it raises the base octane. E10 will therefore be about 3 points of that octane number they state. Let's not forget that there are really 2 issues here, octane and RVP (Reid Vapor Pressure) At altitude if you have an engine that is not turbocharged then extra octane serves no real benefit. I am aware that some will argue that point but if you have ever been to high altitude states like Colorado you will notice that for the most part regular auto fuel at the pump is only 85 AKI and not 87 AKI as we see in states with lower prevailing attitudes. This is simply because the pressure in the cylinder is less due to the decreased ambient pressure of the air. (less molecules per cubic ft of air, less dense) What we need is the octane for the engine to meet or exceed the number in your operators manual for your engine. Turbo engines need a higher number as do high compression engines for the most part. AVGAS is not rated the same way we do auto fuels. AVGAS is rated by its MON value only. If it was rated to an AKI value it would most likely be 105 or more in AKI.
The real concern with altitude is the vapor pressure (RVP) in that with the reduction of atmospheric pressure at altitude the risk of boiling is much higher. The good part about high altitude is the air tends to be much cooler and that saves us to a point. The problem is that some installations may still trap heat, especially in the fuel system, with concerns when fuel temperatures get to critical levels. Winter fuel blends are great for your car. The normal process is to increase the volatility of the fuel by blending high amounts of Butane. Butane is inexpensive and will cause the fuel to vaporise much easier at low temperatures. The problem lies when the RVP number of winter fuel is too high vapor lock can occur. AVGAS has a fixed RVP level that it must be made to meet. You can see the issues when you have 100LL in an aircraft and try to start it in the cold. Preheat the engine is the solution for an aircraft in the winter. (keep it in the hanger at least)
Just to try and put this in a summary. Winter fuels can be as high as 14 PSI RVP. The EPA allows exemptions and can allow 1 more PSI if the fuel has ethanol so may be as much as 15 PSI (high numbers mean more fuel turns to vapor) Summer fuels can be as low as 7 PSI RVP. Summer fuels for autos change by a mandated date in most states however that is not something we can rely on any longer. States, even down to the county within the state, can have varying levels of RVP. The normal time to switch to the summer fuels is often allowed to slip will into the hot temperature months. (political manipulation to sell off the winter blends with no concern for our problems with engines)
So with thinking about all of this what should you run? If you fly over 8000 ft altitude density (some say 6000) then use AVGAS. You have no issues as the fuel RVP and Octane are controlled by the standards to D910 spec for AVGAS. If you want to used auto fuels then use only summer fuels, the problem is there is no way to be absolutely sure you are getting summer fuel. I suggest you build a relationship with your regional distributor of your favorite gas company. (Sheel, Chevron or whoever you like) Find out the dates they switch over. Mixing fuel like auto and AVGAS is fine, 50/50 is simple to do but I would do as you suggest and put some Decalin to help reduce lead buildup in the engine. Remember as long as you are running normal numbers on fuel burn and not using extensive carb heat (carb engines obviously) lead is not normally an issue.
I included a link to a Car and Driver article that is a nice simple primer for RVP for you.
Cheers
https://www.caranddriver.com/news/a15339380/the-vapor-rub-summer-versus-winter-gasoline-explained/
