Conflicting Requirements Re:Engine Oil Viscosity

OldGuy

I'm preparing for my 1st Z oil change at 1200 miles. I'm going to use regular oil for her oil changes until 10,000 mi. The Nissan Owner's Manual prescribes 5 W30. At the same time, I'm preparing to take my Miata out of winter storage and change its oil. Its owner's manual says if outside temps are mostly over 40 degrees F and essentially summer-like, 10 W 30 should be used, 5W30 for climates offering 40 defree F and colder temperatures. My Z will enjoy temperatures over 40 degrees during this oil use period, as summer is one the way. 80 and 90 degrees will happen. I plan to obey the Nissan Owner's Manual and use 5/30 but the Mazda book does provide sensible guidance in distinguishing between warmer and cooler climates when selecting viscosity. Could someone with more mechanical knowledge than I (which would be just about anyone) please clarify this for me.