What did Samuel pour on Saul's head when he privately anointed him ruler?

Answer: A flask of oil

Samuel took a flask of oil, poured it on Saul's head, kissed him, and said the LORD had anointed him ruler over His inheritance.

“Then Samuel took a flask of oil, poured it on Saul’s head, kissed him, and said, “Has not the LORD anointed you ruler over His inheritance?”— 1 Samuel 10:1 (BSB)