Can you mix 5w 30 with 15w 40?

Yes, you can mix 5W-30 and 15W-40 motor oils, as they are both compatible in terms of base oil and additives. However, mixing oils with different viscosities can result in a final product that doesn't provide optimal protection or performance, especially in extreme temperatures. It's generally best to stick to one viscosity grade for consistent performance, as specified by your vehicle's manufacturer.