This question also has been asked here (in Ukrainian):
Як правильно написати в заяві "директору" чи "директорові"?
There are somewhat ambiguous declension rules of foreign proper names in Dative (and, to some extent, Locative) case.
For example Nominative Trump: would the Dative be Трампу or Трампові?
Update. AFAIU, both variants are formally allowed (I have no proofs, however).
However, the latter one
-ові seems to be less used in mass media.
I'm trying to figure out whether or not both variants are, indeed, grammatical.
Note 1. I'm not asking about Noun-based Possessive Adjectives like Трампові лестощі.
Note 2. I'm pretty certain that the Dative for Дональд ("Donald") can be either Дональду or Дональдові.