1. What does me'ila do
Me'ila is the act of using something holy for one's own benefit, in a way that causes the object to deteriorate (by at least a peruta). Once this is done, the transgressor has committed me'ila, and the holy object is desecrated. (This is not true by two things: k'lei sharet - service vessels [vessels that are holy because they are used as part of the service in the Beis HaMikdash], and animals that are for the mizbe'ach. These two do not become desecrated, even after me'ila is committed.) [See me'ila 5:3] Another type of me'ila involves changing the ownership of the holy money/ objects.[See me'ila 6:1 and on] Me'ila is relevant, and desecrates the objects, whether it was done accidentally or on purpose. (Ramba"m me'ila 1:3)
Therefore, assuming me'ila is applicable to the menora oil, it doesn't matter whether the Hellinistic Jews meant to desecrate the oil or not - it would be desecrated.
2. Does the menora's oil have k'dusha
Rashi (menachos 100a) says that the oil for the menora had k'dushas haguf - inherent holiness.
Tosafos (T'mura 14a) disagree and say it never has k'dushas haguf, (rather it has k'dushas damim - monetary holiness), while Tosafos (menachos 89a) say that it had k'dushas haguf once it was placed in the jug that was used to pour the oil into the menora (*which is presumably not the same jug as the one the oil was stored in, seeing as this jug was a k'li sharet and made of gold). This also seems to be Rashi's understanding.
The Meiri understands that the oil only gained kedushas haguf when it was placed in the menora.
R' Chaim Kanievski said that the oil was bought was the Machatzit Hashekel - the half shekel coin donated to the Beis HaMikdash by eveey Jew every year, which means the oil had k'dushas damim as soon as it was bought, even if it didn't have k'dushas haguf until later. (Though there are Achronim who disagree with R' Chaim).
3. How one would be mo'el the oil
As already mentioned, animals that are meant for a Korban and vessels that are used in service do not lose their holiness even after me'ila. The Ramba'm (me'ila 6:5) says that this is true of anything which has k'dushas haguf. However, the Ramba'm (me'ila 6:4) says anything that belongs to bedek habayis (meaning, anything with k'dushas damim) is desecrated after me'ila.
Therefore, if the oil was k'dushas haguf while in storage (as Rashi might hold), me'ila could not desecrate it. If it wasn't, and it had k'dushas damim while it was in storage, as R' Chaim holds, then me'ila is applicable.
Even so, the Mishna l'Melech (me'ila 6:4) goes on a very lengthy explanation of different opinions in the rishonim as to when me'ila causes something with k'dushas damim to become desecrated.
• The Ra'avad (on the Ramba'm) holds that it always loses its k'dusha.
• Tosafos [and the Ritv'a] hold that only the part that one uses is desecrated, unless it changes ownership (i.e., he means to steal it - if it was me'ila b'shogeg, he thought it was his friend's and meant to steal it from him. If it was b'meizid, he meant to steal it from hekdesh), in which case it is completely desecrated. (Note: the Ritv'a seems to disagree with Tosafos about whether he has to have in mind to steal it completely or even if he only has in mind to steal part of it it's completely desecrated, but I'm not sure.)
• Tosafos in Kiddushin holds that if it's a gizbar - someone in charge of the hekdesh, who uses the hekdesh believes it to be his own and therefore only desecrates however much he uses. But others, who know it's not their's, obviously mean to steal and desecrate it all by using even a small part of it (similar to the previous opinion, however here we don't care about individual kavana, rather we "know" that certain people have certain things in mind).
• the Ramba'm has his own opinion in regards to this, which I was unable to understand, see the Mishna l'Melech.
In essence, in order to desecrate the oil according to all opinions above, (assuming it had k'dushas damim; as I've already mention, if it has k'dushas haguf then it won't be desecrated¹, and if it has no k'dusha then this discussion is moot), the Hellinistic Jews would have had to steal the oil. This brings us to the next point: the unbroken seal. I remember seeing one explanation (I don't remember where, unfortunately) of the importance of an unbroken seal was that any closed vessel that they found in the beis HaMikdash was opened to see if there was anything of value inside. Meaning, anything they saw, they picked up in order to steal anything inside, in which case that would be me'ila.
Another possible explanation, is that a shinuy (changing an object) is considered enough to steal it. Therefore, by breaking the seals, the Hellinistic Jews stole the vessels and the oil within, thereby committing me'ila.
- There are some Rishonim mentioned above who hold that there are ways to desecrate something even if it has k'dushas haguf, but I didn't bring them because this is already confusing enough. See the Mishna l'Melech.
Note that me'ila doesn't make anything "impure" as the gemara states happened to the oil. It renders the vessels "unholy". This is something that the Maor HaGadol seems to ignore.