I have a 2017 Elantra (nine months old). At my first oil change, the dealership (without my knowledge or CONSENT) out in semi-synthetic oil (which is NOT required or necessarily recommended by the owner's manual.)
I want to change BACK to regular oil, but I understand that I must flush the engine first.
Should I flush it? Is it safe?
Regular oil as in dino oil? I wouldn't bother. Synthetic oils are usually considered better than dino oil. Many cars don't require synthetic oil, but using a better oil than recommended usually won't lead to problems.
Flushing the engine is unnecessary. If you want to change all of the oil, just drain the old oil out and put new oil back in. Some small amount of the old oil might stay there and if the oils are substantially different they will not always fully mix. This isn't a problem or else we would have a major number of cars suffering from problems where somebody topped oil with a different type of oil due to oil consumption.
The dealership usually know what they do. In this case, they used oil that will not damage your car engine.
(Expanding on other answers, but this won't nicely fit in a comment:)
It's worth noting that many manufacturers - Nissan comes to mind, I think Honda as well - specifically say to never perform an "engine flush." There are a number of documented cases of these flushes causing engine failure.
On higher mileage motors, the flush can loosen up debris which can then block oil passages, gum up the oil pump, etc. (My G35 threw a rod less than a thousand miles after having it flushed. I read about all the caveats a little too late.) On a 2017 model - I'm assuming relatively low mileage - I doubt that would be an issue, but on a low-mileage engine it also wouldn't be necessary.
To reiterate: if the manufacturer says semi-synthetic, do it, or full synthetic. There's no reason to go with 100% dino oil other than to save $100/year on oil changes. Considering you just paid for a brand new car, it seems silly to shorten its life for such a small price difference.
