Some soaps people use to mount tires will etch aluminum. Sodium hydroxide is one soap ingredient that will do it. It's not washed off, but highly concentrated and remains there to eat away at the aluminum rim and/or get chalky. Ever seen aluminum shower and bath enclosures eaten up by soap and water compounds? Those are diluted. Think about what remains behind on your newly mounted tire when a dealer uses something cheap and not really designed for tire changes and highly concentrated.
Alcoa (aluminum) has this in their wheel data: "Before mounting the tire, apply non-water-soluble grease on the bead sheet (rim) of the wheel and tire bead. Water-soluble grease causes corrosion on rim surface, so it should not be used. It is very important to use non-water-soluble grease to keep a good seal between tire bead and rim and prevent air leaks. Alcoa recommends and offers ALgrease as for this application."
That's why I like the Yamaha Tire Mounting Lube stuff, Murphy's Tire Paste, or similar rubber lubes: Slippery at first, then tacky and sticky a few minutes later.