answer:I’ve never heard about it being bad for cars, but I always tend to get lower gas mileage whenever I use Arco gas. Not huge, but noticeable, and enough that the lower price isn’t usually worth it. Mobil seems to be the best compromise for me; I get good mileage, and it’s usually less expensive than Chevron or Shell in my area. Then again, I don’t drive my car much at all. I usually ride my bike, so I haven’t filled up lately.