Mazda recommends fully synthetic for its SkyActiv engines and 5W30 is one viscosity that is listed for this engine. It’s also worth noting that the Mazda engine requires oil that conforms to API (American Petroleum Institute) SL, SM or SN. Although these gradings have an American tag, they’re pretty universal in the industry and denote an oil that is up to a certain minimum standard of protection required by car-makers (Mazda included).
But before I bought any oil, I’d contact the workshop or dealership that did the most recent oil change or service and ask what brand and grade of oil was used. That way, you’re not mixing oils. Different oils have different additives, and mixing them can sometimes cancel out the benefits of these additives.
There may be those reading this who will question whether a new car like a 2019 Mazda should be needing top-up oil between oil changes. But it’s a fact of life that even modern, high-tech engines do consume a little oil between services. Modern all-alloy engines are possibly even a little oil-thirstier than their old-school, cast-iron predecessors. A smart car owner still makes a habit of dipping the engine oil every Saturday morning.
I’d also recommend using a quality, brand-name oil rather than the cheaper, supermarket-branded stuff. Oils ain't oils? Definitely not.