Will it hurt to mix SAE 30 oil and 10w30 oil?

Mixing SAE 30 oil and 10W-30 oil generally won't cause immediate harm to your engine, as both are compatible in terms of basic lubricating properties. However, SAE 30 is a single-grade oil, while 10W-30 is multi-grade, meaning the mixed blend may not perform optimally under varying temperatures. It's best to stick with the oil recommended by your vehicle's manufacturer for optimal performance and protection.