Gravel is an actual D, unlike Benning, thus McLellan preferring one over the other.
I don't know how Gravel would be waiver exempt.
He's waiver exempt in the sense that he has already cleared waivers once so doesn't need to clear them again so soon after having called back up.
Benning would need waivers. Although that realistically wouldn't be a concern. There's little chance he'd be claimed. Bigger problem is likely that you put Benning down in the minors at that is about $900k that sits on the cap and 1.9M you're paying a player down in the minors. Gravel goes down and doesn't count for anything against the cap and only costs the Oilers $300K in actual dollars.
Those aren't big concerns for me. But I could see why its an issue for the Oilers.
There is also something to be said for sticking with the player who you've been trying to develop for the better part of 2 years rather than ditching him less than 10 games into the year after some pretty significant struggles.
Don't get me wrong. Benning has been awful so far. Wouldn't be in my lineup. But I get why the team is giving him another kick at the can.