Some modern engines are really prone to sludge, even with 3,000 mile oil and filter changes. We had a Toyota Sienna van about 15 years ago that was notorious for sludge and huge battles between Toyota and owners. Toyota would invalidate warranties if you were 10 miles over the oil change interval requirement.
The Toyota problem was a special case, and possibly due to improper PCV. Luckily, the modern engines that are prone to sludge are few and far between.