Can I use synthetic oil in my 2023 car if manual says conventional?

Why would Honda specifically recommend conventional if synthetic wasn't safe? You can absolutely use synthetic 0W-20 in your 2023 Civic - it's not going to hurt anything, and you'll likely get longer intervals between changes. The manual's recommendation is usually a minimum spec (conventional works fine), not a ceiling, so going synthetic is actually a solid upgrade if budget isn't a concern. It's not pure marketing, but it's also not a miracle - you'll see real benefits in longevity and performance, just not dramatic engine transformation.

Synthetic 0W-20 will work fine in your Civic - Honda's just being conservative with the manual since conventional is cheaper and meets the specs, but synthetic absolutely won't damage anything. The longer drain intervals are real (maybe 8,000-10,000 miles versus 5,000-7,000 with conventional), so you'd actually save money over time despite the higher upfront cost.

One thing nobody mentions: if you switch to synthetic, stick with it. Mixing or bouncing between them isn't harmful, but you lose those extended drain benefits if you're topping off with conventional between changes. Just grab a 0W-20 synthetic jug for top-ups to keep things consistent - most auto parts stores stock it and it's only a couple bucks more than conventional.

Your Civic will run just fine on synthetic 0W-20 - the viscosity rating is what matters to Honda's engineers, and synthetic meets that spec perfectly. The manual lists conventional mainly because it's the cheapest fluid that gets the job done, not because synthetic would somehow damage anything. If you're keeping the car long-term and don't mind the extra cost, synthetic's longer drain intervals will probably save you money and hassle over time.