Charles Ellicott Commentary Genesis 28:18

"And Jacob rose up early in the morning, and took the stone that he had put under his head, and set it up for a pillar, and poured oil upon the top of it." — Genesis 28:18 (ASV)

Jacob ... took the stone ... and set it up for a pillar. —In so doing, Jacob’s object was to mark the spot where so important a communication had been made to him. But besides its use as a memorial, it would enable him to identify the place upon his return, and pay there his vows. And as oil was the symbol of the dedication of a thing to holy uses, he pours oil upon the top of it.