Torah Temimah cites Toras Kohanim.
“For it is a chatas” — he shall not put oil, and if he put it, it is invalid; might one say the same for frankincense, that if he put it, it is invalid? Scripture says “for it is a chatas… it” (limiting the harsher rule).
Ribbuy / miyut of “chatas hi” — oil vs. levonah distinction.
Adding levonah is forbidden but does not invalidate in the same absolute way that oil does. The chatas identity of the flour is preserved unless oil is mixed in. The Torah’s language therefore treats the two substances under different levels of severity.