He wanted to make a Flash Gordon adaptation, not Dune. It shows, too, with Star Wars' aesthetic being heavily inspired by both Flash Gordon and Buck Rogers.
Narratively, the original trilogy is heavily influenced by pulpy sword-and-sorcery books and comics and Kurosawa's films, but with the twist that it's in space.
Dune, meanwhile, is Hamlet crossed with Lawrence of Arabia in space.
Spice appears in both, except what spice actually does in Star Wars wasn't explored in anything Lucas made and Frank Herbert died before any of the expanded universe existed. There are only three mentions of spice in the entire original trilogy, all in ANH, and those are a bit about Luke believing his father was a navigator on a spice freighter, 3PO mentioning the spice mines of Kessel, and Han having to dump a load of smuggled spice. It's clearly just a shout-out; the spice is just a background reference and doesn't feature in the story. You could replace spice with beanie babies and nothing would change.
What parts of Star Wars do you feel originate from Dune? I've never actually gotten a straight answer and I'm genuinely curious.