Use What The Car’s Manufacturer Recommends (Requires). It’s Often Written On The Engine’s Oil Fill Cap. Better Yet, Look In The Owner’s Manual.
Sometimes, there are temperature ranges for different viscosities. You could then use the “heavier” oil year-round in Florida.
Example: Use 5w30 oil at temeratures above 10F.
Example: Use 5w20 oil at temperatures below 10F and up to 100F.
Thinner oil helps your car at cold starts, but your car’s cooling system should keep the engine operating at similar operating temperatures, regardless of where it’s driven and what climate it’s in, once it’s warmed up.
I’d go with the engineers rather than good intentions.
I’ve had several times the mileage you’re talking about on many cars, just by using the recommended oil.
CSA