the Owner's manual tells us to use 10W-30 oil, what would happen if one used 10-40? will this do any harm and why?
GN-4 10W-30 or better. As my local dealer and people whom I know having years more experience have said. Use the brands oil and filters, keep the receipts, go by the recommendations in the manual for changes and they have no reason to deny any warranty claim.
I asked about 10W-40, wondering about operating the bike in higher temps like Death Valley and they said yes, do that. I have now been using the 10W-40 Honda blend (non molly) gold bottle/red cap. I know......over kill.
My dealer thought I was throwing money away. It was a funny exchange when I was there talking this over. He tells me.....why pay that much for oil? I question him......that much?! It's only like $8ish a bottle? He said.....yeah, that's a few more dollars then you need too. I thought.....that's cheap!

He looks at me and says......wait, what kinda bike were you buying oil for? I tell him.....BMW and a Husqvarna. He starts to laugh and says.......Oh, you're one of those guys. (LOL) You just want to pay double for everything......yeah, I get you guys in here every so often, looking to spend more money on everything just because you're used to it!

I laughed.....and told him he was right. I was used to spending like $12-$18 a bottle for oil so even the Honda blend seemed like super inferior crap. My dealer goes on and says.....I bet you Honda tested the crap out of this motor on basic 10W-30 non synthetic, bottom of the barrel oil, knowing the general public wouldn't pay for better.
Anyway, not to start a crazy oil debate......put what you like in it, it's your bike but you can't go wrong with basic 10W-30 or for highter temps 10W-40.