bcbristol, those old rating numbers are far different than todays. The 90 was an leaded regular grade of gasoline good for up to about 8:5 to 1 compression where as the new regular unleaded is good for up to 10:1.
Our A-4's are at 6.5:to 1 so finding a good fast burning fuel is problematic. Your best bet is todays regular unleaded without the ethanol if you can find it. If we could find some 83 rated regular things would be great.
You will make more HP and torque with regular fuel and such low compression. Running premium is a waste of time money and horse power.
Dave Neptune