Why does the Torah write that Yakov took "the stone ... ", when initially, it was a number of stones that he placed at his head?
What is the difference between a Mizbe'ach and a Matzeivah?
Ramban: a. Whereas the former is made of many stones, the latter comprises just one stone. b. Whereas one brings Korbanos on the former, on the latter one pours wine or oil. 1
Matzeivos became forbidden when they entered Eretz Yisrael and Mizb'chos permitted, because the latter were not as popular as Matzeivos among the idolaters, or because the Torah did not want to prohibit everything.
Why did Yakov pour oil on top of the Matzeivah?
Rashi (in Vayishlach), Rashbam and Seforno: To initiate it for use - to offer on it Korbamos upon his return journey (twenty years later). 1
Ramban: Refer to 28:19:1:1.
Why did Yakov not pour wine on the Matzeivah, like he did on his return journey?
From where did Yakov get oil? Elifaz took all his money, and Yakov left Eretz Yisrael with only a staff (Bereishis 32:11)!
Tosfos ha'Shalem (1, from Pirkei d'R. Eliezer 35): "Es" teaches that oil descended for him from Shamayim.
Pane'ach Raza: The staff was hollow and full of oil, so Yakov could light it and learn at night. 1
Even if he could make a fire from sticks or rocks, where did he get a lamp? Also, there were no Seforim at the time. Why did he need light in order to learn? (PF)
How could Yakov make a Mizbe'ach from a stone that a person used? This is forbidden (Zevachim 116b)!
Tosfos ha'Shalem (4): One may make a Bamah from what a person used.