Is 3,000 miles really the oil change rule or is it just a scam?

Car manuals say 3K miles, but my mechanic laughs when I bring it in that early. "You're burning money," he says, but I'm still paranoid. My dad swears by 5K and my brother does 7K with synthetic. Who's actually right here?

I drive a 2018 Civic mostly highway miles—no extreme temps or towing. Still, every 3K oil change feels like throwing cash at the dealer. What do you think?