"I am in Kyiv like cheese in oil."

Yes, "in butter". "Кататися як сир у маслі" = To be rolling like cheese in butter.

Made "butter" default, "oil" still accepted.

Thinking of adding a non-literal translation e.g. "rolling in dough"... But need to consult with the others. E.g. "I'm rolling in dough in Kyiv" (that changes the word order, so not so nice...)