Is it safe to use 5W 30 and 5W 40 oil in same engine?

Using 5W-30 and 5W-40 oil in the same engine is generally safe, as both oils have the same viscosity at low temperatures (5W). However, the higher viscosity of 5W-40 at operating temperatures may affect the engine's performance and fuel efficiency. It's best to stick to one oil type and viscosity recommended by the manufacturer for optimal engine health. Mixing oils can lead to unpredictable results, so if you're unsure, consider a complete oil change with the recommended viscosity.