运油-20,in Romanized pinyin Chinese = Yun You-20 (= YY-20), this is official designation in Chinise language though in English it may be different.
Since H-6U is considered as the "official" designation in Romanized letters, 运油-20 would be more appropriate be written as Y-20U instead of YY-20.
On the other hand, are we affirmative that H-6U is the confirmed official designation and not a designation given by China military watchers?