Oil embark good upgrade at experience level (6) where you can't take it (need level 7 upgrades)

[Aschere]

You unlock the oil embark bonus at level 6 which costs a whooping 4 embark points. Unfortunately even if you took all upgrades through level 6 you will not even have enough embark points to take oil with you. You need to also get the level 7 upgrade of it to be of any use.

I'd recommend moving the oil embark level up to level 7 otherwise its useless other than people may waste points only to find out you can't use it. Or put in a warning saying "Requires 4 embark points. Current max embark points is 3" or something.

[DG]

The cost varies on embarkation and sometimes it can be 3.

[Narandia]

You technically only need one embark points upgrade to afford it... on Settler. Higher difficulties take up to 2 points away.
Could perhaps be structured a bit more flexibly (so you can still take embark bonuses for resources that are normally outside of your available total, just proportionally less), and I'm not a huge fan of the random cost variation either, but its not exactly a logical flaw in the upgrade tree structure.