The H8 / Group 49 big boy battery is what you want for the diesel.I tried to lookup a fitment for my 2017 Diesel and Optima’s website says none exist. What battery did they tell you works? I saw their write up about Pureflow but nothing more.
EDIT: I dug some more and found it. But it is not for the diesel, only gas. Oh well. I am still looking a good AGM.
Costco is weird this way, they literally did not want to sell me a battery for my kid's Mazda 6. As long as the battery physically fits in the tray and at least meets the CCA spec of the OEM battery it will work, it is not rocket science. Finally got Costco to sell me the battery if I agreed to void the warranty. That guy really did not want to sell the battery. I persisted as I like the Interstate batteries they carry at Costco, and it was about $50 less than the "official" battery at AutoZone. Whatever, the car starts and runs. That is all I care about