I remember learning the metric system in school in the 70's. "Get ready America!"
We are way too stubborn for that. Besides, nowadays converting to the metric system would be seen as one more step toward World Gubment. "How many liters of fuel does a Black Helicopter need to get from it's secret base to your hometown?"
Seriously though I lived in Australia for a year, and didn't find it too difficult.
I wouldn't be surpsied if the military is already using metric (in part maybe because of NATO) and isn't aviation fuel measured by weight ie. Kg. As you need to know the weight of an aircraft including payload, to see if the plane will generate enough lift to take off.