HD's service procedure for my 2018 RGU requires changing the OAT coolant every 30,000 miles, which is way sooner than required by the auto and heavy truck manufacturers--they are anywhere from 100K miles (auto) to 1 million miles (heavy diesels) (although the hauling industry requires testing of the OAT coolant for proper levels of additives). I will stick to the 30K requirement, though, just to maintain the warranty.
I've also bought the Peak Global Final Charge 50/50 OAT coolant to top up the reservoir. It's one of many that are approved by the industry as meeting all the standards, and it's readily available through AutoZone--in fact, several of my local stores keep it in stock, and at a very reasonable price. The nice thing about this coolant is that, as long as the current coolant is good, there's no drain-n-fill needed; just start using the new stuff to top up.