Basically a cargo ship can hold 1000 x10 x50 = 500,000 units of oil with barrels while the tanker ship only has a capacity of 150,000. To me it feels like tanker ships should hold around 600,000 units but that is up to you.

As a point of reference, a vanilla cargo wagon with barrels will hold 20,000 fluid, while a fluid wagon holds 25,000. If we try to mirror vanilla balance it should hold 625,000

well... maybe the capacity off the cargo ship is just overly exessive. But i am afraid if i change this i might break peoples setups. And at least in my games it already takes forever for an oil rig too fill up a tanker as is. But i will consider maybe doubling the tankships capacity too bring it more in line with the cargo ships volume

The ships are also much, much slower than the trains they are based on. By the time an oil tanker makes it all the way to my harbor, the oil trains have delivered 8-10 trips. A factor of 2-4 would be fine, but 8-10 is excessive. Which is just a way to say that the oil tanker should have 2-3x the capacity.

I've found 625k to work well for me. Sure, it takes a long time to load, unload, and even obtain that much oil in the first place, but that is dwarfed by the amount of time it takes for the tanker to get from A to B.

A round trip takes a very long time. These kinds of ships are extremely slow for a very simple reason: they carry stupid amounts of stuff. 150k isn't enough to warrant the speed it moves at by a very long shot.