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That is their recommendation , you really need to see what the handbook says as you could potentially damage the engine with the wrong grade oil in , Honda tolerances are tight so needs a thinner oil , hence 5w20
15w-40 means it behaves like a 15w oil at cold temperatures and a 40w oil at high temperatures.
So it's a good bit 'thicker' at all temperatures than a 0w-30.
A 0w-30 oil will give you better protection when starting the engine in cold temperatures.
I wouldn't really recommend you use a 15w-40, unless it's an emergency.
God no. Modern engines need the correct oil, 15w-40 is for old engines and I wouldn't put it anywhere near any Honda engine! If it was an emergency top up then fair enough (any oil is better than none) but don't scrimp on good oil for a full change or normal top-ups. Go to civinfo.com for all the info you need!