Jacob set up a stone pillar at the place where God had talked with him, and he poured out a drink offering on it; he also poured oil on it. Genesis 35:14 (NIV)
This passage marks the second time in Scripture that oil is mentioned, and both times, it is done to mark a specific pillar of stone that Jacob set upright. The first time was back when Jacob saw the stairway to heaven, with angels ascending and descending.
Early the next morning Jacob took the stone he had placed under his head and set it up as a pillar and poured oil on top of it. Genesis 28:18 (NIV)
In both cases, it can be assumed that Jacob is marking a place to remember. The upright stone would stand out, and the oil would mark the stone in an unnatural way, allowing Jacob to return and know exactly where he was when the miraculous encounter occurred.
The writers of the text would hope that you notice these markers as well. And they would likely suggest that this is a place where you should mark something to remember what you are reading here: Poured or smeared oil is where we get the word "annointed." It's where we get the word "Messiah." The narrative between these two markers is profoundly important.