Looking at www.glosbe.com/sw/en/mafuta , it seems to be both. Oil, fat, grease, petroleum, cooking oil etc. In the translation memory, I can see it used for the oil that birds preen their feathers with, oil for annointing, oil as in oil spills, palm oil ... so basically anything that the English word "oil" means.
Mafuta is a plural-only word, and comes from Proto-Bantu *kuta. Cognate with Bemba mafuta, Luganda futa, Kikuyu maguta, Ronga amafutha, Kua makura, Lingala mafuta, Luba-Kasai futa, Mwezi maguta, Rwanda-Rundi vuta, Sango mafutha, Shona mafuta, Sotho makhura, Tswana mafura, Venda mapfura, Zulu futha.