They are not even slightly similar.
Oil creates fuel, gets put into a distribution pool, the fuel gets distributed across your army but not by rails as supply does.
Fuel impacts air, navy and army. Oil can be traded and modified with infradtructure.
Supply gets created by the supply capital and distributed along the best path along a railway to every supply hub with a supply drainer (army, air supply etc). The supply gets distributed from there via an area of effect. The size of the area is determined by terrain, motorization, crossing rivers and more. Supply cannot be traded but supply does work with your allies, and your allies can take your supply if they don't have any yourself.
Supply trains can get bombed, railways can get bombed, hubs can get bombed. The fuel "container" cannot.
Supply and equipment isn't the same either. equipment is created by mils, supply by the supply creators (supply capital, large cities, infra etc)
Supply is also created by cities, harbors, distributed across the seas etc. The systems are so widely different on a fundamental level and spatial level that I can guarantee that simply "cloning and slightly modifying it should be possible to modders from a technical standpoint." Is not what we have done. The programmers ln the team has done a herculean effort in redoing the supply system.
Why isn't it moddable? well performance is one. Some things are just so much faster and systems with lots of calculations might need that.
Complexity is another. There are things that programming languages can do that our scripting language is not suited to do. Complex systems are one of those. Exposing source code is not possible.
We do strive to make as many systems moddable as possible. We put a lot of extra time trying to make them so, because then we end up with amazing thins like OwB, KR, EaW etc.
Sometimes though for several technical reasons those are not possible.
I hope those answered some of your questions
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