I have studied that if the electronegativity difference between O- and element M is more then it is a basic oxide and if the difference is less then it is an acidic oxide. However, it is a fact that higher oxides of Non metals are more acidic than lower oxides. If the theory concerns polarisation, is it not contradictory?
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$\begingroup$ Getting idea about cutting out "acidic oxides" out of every textbook and forcible teaching the authors the Lux-Flood theory, after seeing this... $\endgroup$– MithoronMar 26, 2019 at 23:14
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It is expected oxides with more polar $\ce{M-O}$ bonds are stronger Lewis bases than those with less polar bonds.
Furthermore, for the same element, the polarization gets weaker/Lewis alkality weaker when more $\ce{O}$ atoms are attached.
When hydrated to acids, another phenomena plays a role in acidity: a charge delocalization.
It is well observable that acids like $\ce{HNO3}$, $\ce{HClO3}$, $\ce{HClO4}$, $\ce{H2SO4}$, $\ce{HBF4}$ are strong, as the delocalization has a good sandbox to play in.
It also plays role in solubility of salts in organic polar solvents, where there is a large organic cation with one of the above small anions.
