What happens if you mix 10w30 pennzoil with 10w40 pennzoil?

Mixing 10W-30 and 10W-40 Pennzoil oils is generally safe, as both are compatible and have similar base oil compositions. The resulting mixture will have properties that fall between the two viscosities, typically resulting in a viscosity closer to 10W-35. However, it's important to check your vehicle's specifications, as using the recommended oil viscosity is crucial for optimal engine performance and protection. Regularly mixing different viscosities is not advisable for long-term use.