dont always believe what you find. i pull valve covers daily, and i see sludge all the time. all of them are using as you say "change your oil as recommended with OEM quality filters and oil" 15k miles is way too long for these engines. just because the oil may be still good(synthetic) doesnt mean it doesnt get dirty, or collect fuel from blow by, or pieces of the oil filter come apart because the engine oil runs at 210degrees. And then when heat soaking, the oil and dirt particles stay on top the the valve train causing gunk or sludge to build up. I change my oil every 3,000 miles. when i pull the valve covers off, you can still see the machining surfaces of all the moving parts.