You can mix a 5W20 with a 5W30 oil with minimal amount of issues provided it the same manufacturer and the same API classification.
Can I mix different 5W 30 oils?
Oils get mixed all the time and it's not a problem. It is perfectly ok to mix them. You won't get the maximum protection of the best of them, but all motor oils are compatible. Actually, every time you change brands you are mixing about 15–20% of the old oil with the new that you are using.
Is it bad to mix motor oil weights?
It is always advisable to not mix motor oil brands, however, different viscosity grades of the same brand motor oil are compatible. Be aware that mixing viscosity grades will turn out a product that is different in viscosity than either what was originally in the engine or what was added."Nov 15, 2018
What happens if I use 5w20 oil instead of 5W-30?
It flows as 5W cold and flows as 20 weight when hot. So in effect the 5W-20 flows thinner when hot than the 5W-30.
Will it hurt to use 5w20 instead of 5W-30?
Probably not. 5w20 oil is fairly light oil and is generally designed to work with newer engines. Motor oil has become part of a manufacturers program to allow engines to get higher fuel mileage. Lighter oils take less HP to pump through an engine than heavier oils.
What do I do if I put the wrong oil in my car?
Use only what the owner's manual specifies. Using the wrong oil can lead to reduced lubrication and shorter engine life. If the manual says to use synthetic oil, do so. Contrary to what some believe, adding synthetic oil to regular oil won't harm the engine, but there's also no benefit in doing so.