However, if your car normally requires the use of 5W-20, it's dangerously low on oil, the only motor oil you have available is 5W-30, and you must drive it, it's better to add the 5W-30 than it is ...

It's acceptable to use 0W-20 engine oil in place of 5W-20 or even 10W-20, since the number preceding the W refers to its low-temperature viscosity. However, you should match the oil grade exactly ...