[Cardinal7167]

Go even has a mantra for this: “accept interfaces, return concrete types”. In the example, I would think to design it such that Oil is just an interface with adapters for either user specified oilType or oilInventory or whatever.

[suprfnk]

That seems like a more specific version of Postel’s law/the robustness principle:

“be conservative in what you do, be liberal in what you accept from others"

https://en.m.wikipedia.org/wiki/Robustness_principle