The only quesiton is whether one wants to roll over and buy that line of horsechit or not. Google The Magnusson Moss Warranty Act and read a bit about it. The bottom line is that unless Harley gives the oil away, they cannot condition warranty coverage on use of their name-brand oil.
It comes down to whether one wants to allow themselves to be "legally abused" by a manufacturer who decides not to follow the law unless forced to by a lawsuit or an attorney general picking up the cause on behalf of the public. Please understand that I'm not saying that's Harley's M.O. in the least.
But I do see companies every day that play those odds--if they have 1000 warranty claims for a known problem, is it more cost effective to deny coverage and take the risk that 3 of the customers will actually do something about it and sue, rather than pay out on the other 997? They figure 2 of the 3 will settle for small amounts, and if they get hit on the 3rd one they're still money ahead. On products like tv's and other consumer items, people generally don't get injured or suffer property damage if warranty coverage is denied, so it's purely an economic decision. Of course, the negative PR they may receive also plays into it. But it often has little to do with simply making the customer happy. Sometimes it does, but not always.
I have never heard of Harley denying warranty coverage based on oil selection. And I personally don't think they could do so within the bounds of the law. Your dealer's advice is to avoid the fight in the first instance, and don't use anything but Harley products. Not that he has a financial incentive to make that recommendation

But just be careful if you decide to put Custom Chrome mirrors on the bike. That may be the reason used to deny coverage for that bent valve.
OK, cynical rant off.