The new gasolines have little to no residue deposits on the spark plugs or combustion chamber and valves for that matter. One of the frustrating things is you can't effectively use the spark plug insulator color as a gauge for if the engine is too lean or rich, like we used to do with the old leaded gasolines. Usually you can pull a plug out at 50,000 miles and it doesn't look like it had any wear on it at all, or very little wear, and the electrode looks clean with little to no color to it. Thus the spark plugs don't foul like they used to, unless the engine has bad rings and starts using oil and smoking.