Most car fuels today are in the oil content range of 9% to 12%. One maker does makes a 16% break-in fuel for car engines. When the Cox format engines were designed, airplane fuel was 25% all castor. This level worked fine as it prevented the rod big end from galling on the crankpin, And it also kept the rod ball and piston socket from wearing out prematurely. They also required a higher concentration of nitro in the fuel which usually standardized at 25% to get a smooth even run. I used to run TD's in C/L Combat with nitro content as high as 45% and if I didn't add castor to the 18-20% oil fuels available in the 80's, I would find pieces of the rod "welded" to the crankpin.
Your can use the car fuel, but your going to have to add castor oil or Klotz Benol to it. Easiest way to go to the Walmart pharmacy and buy a 6 ounce bottle of castor oil ($3.00 or less) and add it to a quart of 30% car fuel. If your going to fly a lot of 1/2A, go to a motorcycle shop get the Benol. It runs about $9.00/pint. To get specific, if you get 9% fuel, add 6.75 ounces of castor. If the 12%, add 5.5 ounces. This will get you to the 25% mark in oil and around 25% nitro. You can modify the 16% break-in fuel with less castor/Benol, but you will be below 20 % on the nitro.