Glad you said something about the the PCV valve, My brother bought a brand new Ford Excursion last year and when they brought it in for service the found it was almost 2 quarts low of oil. They topped it of and kept an eye on it and it was a new vehicle using oil. Needless to say my brother was pissed! He then made them keep it to find out what was going on and in his case the PCV valve was defective causing it to use oil! I agree , Always use use distilled water in radiators to keep the chlorine and minerals from breaking the coolant down! I run Pennzoil full synthetic in my 2018 sonata and have it changed at the dealer, I furnish the oil and they furnish the filter and service. Bought it new in 2018 but only have 16,000 miles on it. I could hear the motor get quieter after the first oil change at 5,000 miles.The engine and cabin air filters are super easy to pull out and check on the Sonatas. Just switch them out whenever they look dirty regardless of mileage. I have a paint brush in the garage that I use to remove leaves and any larger debris from the filters between replacements.
In terms of transmission fluid, I plan to change it at 60,000 miles just for peace of mind sake.
I replaced the sparkplugs on my 2011 Sonata at 60,000 miles even though they still looked great. No difference in performance or gas mileage. On my 2019 Sonata I will check them at 60K and put them back in if they still look fresh.
One big thing to replace that is not mentioned by Hyundai is the PCV vallve. At 60,000 miles on the 2011 it became clogged and I began losing oil on highway trips. Cheap and easy to replace yourself. Check YouTube for the procedure.
When you replace the brake pads also flush and replace the brake fluid, if you really want to keep the Sonata going trouble free for a long time
I had my cooling system flushed and the coolant replaced at the severe service interval at the Hyundai dealership. The service tech chuckled when I asked him to use distilled water (which I supplied) instead of tapwater to add to the coolant. This helps the additives in the coolant to last longer and reduces scale in the system. It is hard nowadays to tell when a hose is about to fail due to age, so anything that keeps me off the side of the road is worth a little effort.
If this extra maintenance sounds a little expensive, it is initially. I've done spreadsheet comparisons on corporate aircraft maintenance and in the medium to long term a well maintained airplane is cheaper to operate because of fewer repairs.
Also, I got at least $1000 more on trade with my stack of dealership service records. Properly maintained cars are rare and go for a premium. The salesman called a few of his contacts looking for a good used car and my 2011 was resold the day I traded it in.