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The 6.4 takes 0W40 not 0W20. I think the 3.6 takes 0W20 the last few years though. Personally I bought around 50 Qts myself of the 5W20 and 5W30 because the price was just to cheap to pass up. Its pretty much all sold out around here now.
Strictly gas mileage and nothing else. I would be running something like 5W30 or even 10W30 in an older high mileage engine for sure myself regardless of the recommendations. Back in the day we ran nothing but 10W40 in American cars and 20W50 in the *** stuff and it worked.
You do realize that 0w30 and 5w30 and 10w30 are all the same weight oil? It's all 30weight oil, exactly same viscosity, not "water". That 0w or 5w is the winter viscosity, it's the cold thickness of the oil, making it easier for your engine to start, and flows faster when cold to lubricate all posts of your engine, where at warm viscosity, it would take longer to to reach all areas. Modifiers in the oil react to temperature change, thickening when warm to make the oil a 30 weight.
It's labeled different because they are different oils for different purposes. The baseline oil used is the same but the additives in the oil are what changed the properties based on temperature so why wouldn't you label it different if it does different things? You seem to think that just because you live in Florida they should make one label for all their oils, you do realize there's a whole world outside of Florida right? It might be hot in Florida, but I can guarantee it's a hell of a lot harder inside your engine at full operating temperature, so why not create an oil that works best at every possible temperature range instead of just one.
