My take.... Higher octane done not automatically mean a slower burning fuel, my understanding is there is no direct correlation. The resistance to detonation is based on certain chemicals not forming during the combustion process that lead to spontaneous ignition, detonation resistance is based on preventing those compounds from forming ( I cant remember their names of the top of my head).

This is not the common belief and I am sure that someone will say its wrong.

One thing worth looking at is the O2 sensor. If I remember right leaded gas is commonly available in Venezuela. If your computer system is designed for closed loop and you get a few tanks of leaded your O2 sensor could be bad, leading to a drop in fuel economy.

Kevin