Carbon deposits on the intake valves of Gasoline Direct Injection (GDI) engines are becoming more common as these engines rack up the miles. The leading cause of carbon deposits on intake valves are crankcase vapors circulated through the PCV system. Conventional oil that has high volatility will cause more vapors and can compound the carbon problem. Synthetic oil with low volatility can reduce vapors and prevent carbon from forming on the valves. Sponsored by Auto Value & Bumper to Bumper.