TL;DR POST INC
There are a few options I can think of. The first one is just using gold/lumber, which you can't, so that's out.
The next option is some kind of region-based marker. When the ship enters a region around an oil mine, it gets marked in some way (mana amount, custom value, item added to inventory) which will be recognized when it goes back to the port / wherever it will deliver the oil. Alternatively, if you want the oil to be automagically transferred, you can do that pretty easily.
Another alternative would be a solution using mana drain spells. The mine would be a unit with a certain amount of mana = how much oil it has. The oil tanker would then use a mana drain spell on this, which would increase it's own mana (which would be cool to use in other ways - let out some oil to mess with the waterways - but I digress).
In any case here, there is the problem of automating the process. This can be done quite easily by saving some information in a hashtable.
Create a hashtable at map start and set that hash table to a global variable.
Then, do this:
Make a dummy spell for the oil tanker called "Select port". When this ability triggers, save the Port unit as a 1 of your hashtable name with the key(triggering unit). This will make it so that if you load "value 1" of the hashtable with your unit as a keyword, you get that port unit right there.
Next, add another spell called "Start harvesting" or similar. When cast, save the oil mine as a 2 of your hashtable, again with the key triggering unit. The actual ability should be a modified Drain Mana ability, apart from its trigger functions.
When your periodic trigger detects that an oil tanker has full mana, order it to use a third dummy spell on the port. When this is detected by your trigger, set the tanker's mana to 0 and add the boat's current amount of mana to a global variable somewhere which you can check against later.
After that in the trigger, order the unit to once again use the second dummy spell (the mana drain) on the gold mine which you stored earlier in the hashtable, and the cycle should close itself.
If you need help understanding any of this, please say, and don't be afraid of using that hashtable - it's pretty easy, really.
