MOGAS Fuel

"So to answer the original question.  Use the higher-grade fuels, it is better protection from detonation."

Except that... the higher "grade" fuel is more likely to have (more) ethanol.  In my area, most stations sell 93AKI, at up to 10% ethanol.  The non-ethanol fuel that's available in my area is unblended (obviously) and thus has an AKI of "only" 91.  I'd much rather use the latter; I don't think I'm in any real danger of detonation.

When testing to see if ethanol was suitable Rotax selected Brazil and found aircraft with almost 1500 hours of running on Brazilian auto fuel.  the fuel was over 22% ethanol and an RVP of 11PSI.  The good part there is the fuel has not summer or winter blends, it is all the same. (unlike in the USA and Canada where they change it seasonally) Rotax will never allow more than 10% as more will always show excessive NOx emissions. The trick is to simply keep it fresh, do not store it for any long periods of time and there is no issue.  

Just to toss this out there, if you are flying any high altitude, say over 8000 MSL, I would personally use Avags with lower RVP to avoid vaporization.  Auto fuels are not blended for higher altitudes.  Modern auto and truck engines also have elaborate fuel vapor separators and devices to deal with fuel vaporization which aircraft OEM do not bother with.  Most OEM rely on using fuels with very low RVP ratings to do that job.

As long as you can get at least 91AKI fuel you should be fine with the carb engines.  Personally, any turbo powered ones I would switch to higher octane or blend the fuel with Avgas.  (yes, I am obsessed with detonation issues on high cylinder pressures)  

The FAA has some great study material on fuels and effects of ethanol and other alcohols in the fuels.  There is a special ASTM committee DO2 that has under its umbrella all the aviation fuel standards.  Depending on where you live you may be able to find UL94, an ASTM compliant fuel from Swift, it has however a very small distribution base and not that easy to find.  It is lead free and it is 94 MON octane so meets our needs without any problem.  

Is there any additive that can be used with mogas to lower the RVP?

I don't think that would be possible.
The RVP is mostly a function of how much Butane is in the mix.
You would need to remove the Butane.  Evaporation???
But then it would be anyone's guess of what the RVP and Octane rating you would have remaining.

Unless you have winter gas and the ambient temp is way above normal, it usually isn't a problem.

At 10,000 ft winter gas is good to around 75°F;  Where is it ever 75° at 10,000 ft where winter gas is being sold?
At 10,000ft Summer gas is good to  100°.
And 100ll is good to 18,000ft at 100°.
But then, the standard Temp at 18,000ft is -5°F and 22° at 10,000.

Using leftover winter gas into the summer is where the problem comes in.
Buy your gas as you need it,  Fill up at the start of a flight, not at the end. Don't store it.  

I don’t store gas. Here In the south it’s often hot in the spring while they’re still selling winter blend at the pumps. I had a problem on an 85F day in March. Airplane had been sitting on the ramp in the sun, so who knows what the fuel temperature was. I had topped off with 100LL, but had flown in with mogas, so probably had a 50/50 mix in the tanks. On takeoff roll, when the turbo kicked in (915) and the engine demanded fuel, I lost fuel pressure. I speculate it vaporized on the suction side of the Rotax pumps. (Low wing airplane.) I’ve been gun-shy about mogas ever since.