Lewis pronounced Aslan "Ass-lan"
In a 1952 letter to a young reader, the author explains where the lion's name came from — and how he said it himself.

On 22 January 1952, writing from Magdalen College, C. S. Lewis answered a young reader, Miss Jenkins, who had asked about the name Aslan. The reply, published in his collected correspondence and in "Letters to Children", is short and exact.
He explains that he found the name in the notes to Edward Lane's English translation of the Arabian Nights: in Turkish, aslan simply means "lion". Then he adds how he says it himself: "Ass-lan", with a clear s, not a z. In the same letter he also notes that he meant the Lion of Judah.
The detail matters because many readers, and several adaptations, have grown used to saying "Az-lan". Lewis does not present his pronunciation as a rule everyone must follow; he simply says how he practises it. On narnia.guide, Aslan's character page already records the Turkish meaning and the Arabian Nights notes. What is less often repeated is this spoken detail from a 1952 letter, given in answer to a child's question.
The letter also shows the care Lewis took with young correspondents: a concrete question, a clear answer, and no digression.
See also
Sources
- The Inklings — Aslan (excerpt from Letters to Children, 22 January 1952).
- Wikipedia — Aslan (citing Collected Letters, vol. III, p. 160).
- Science Fiction & Fantasy Stack Exchange (letter to Miss Jenkins quoted from Collected Letters III).