I had an oil leak from the valve cover in my 07 Kia Optima. Its got 148,000 miles on it, and the gasket was very brittle. It spent the first 90,000 miles on a diet of quick-lube oil changes done every 5k, and the last 48,000 on what ever syn I can find on sale, changed every 10k miles or so. Not too bad, I think. Absolutely no signs of sludge.