Rav Hirsch writes that usage of מנחה as a flour-and-oil offering is imprecise:
The מנחה offering in the Sanctuary is an offering of flour and oil. But we do find מנחה as a general term for offerings - even for animal offerings; thus in Malachi 1:10, 1:13, and 2:13, and throughout the book of Malachi. Outside the Sanctuary, מנחה denotes a gift, a sign of homage.
It is difficult to determine the root of the word מנחה. ... It seems that מנחה stems from the roof נחה, just as מצוה stems from צוה. [The root] נחה means “to lead.” The shepherd leads his flock; the commander leads his army. In short: a leader or commander guides his subordinates. Perhaps this is also the meaning of the מנחה. The offerer submits to the leadership and authority of another, or acknowledges his lordship.
With this explanation, that the term מנחה refers to Karbanos in general, Rav Hirsch explains that the difference between Kayin’s and Hevel’s Karbanos was a difference in attitude:
Kayin brings to G-d מפרי האדמה, some of the produce of the earth, without special selection. After all, he must offer “something” also to G-d. This is an improper attitude, which will later be censured by the prophet Malachi: The relationship to G-d, “religious needs” - they, too, are among the nerds of man, and cannot be ignored entirely. One devoted to them the “lost hours” of life, the lame and the sick and whatever has no other use. As Malachi puts it (Malachi 1:12): וניבו נבזה אכלו; what no one else would wish to eat is given as the altar’s harvest. This is service if G-d that is precipitated by fear. People establish temples and churches in the same spirit in which they build hospitals and prisons.
Hevel, however, takes מבכורות צאנו ומחלביהן; he takes from the firstlings of his flock, and from them he selects the very best. He who offers the first and the best attaches primary importance to his relationship with G-d. This relationship is his foremost concern, to which everything else in life is merely subsidiary. What is more, offering the first is always regarded as a substitute regarded as a substitute for dedicating all the rest. As it says explicitly (Shemos 34:19): וכל מקנך תזכר פטר שור ושה, “All of your herd shall be dedicated by the firstborn of ox or sheep.”