Hello, its me again, with some questions: is this mod compatible with mods that can change the capacity of barrels(example: ReStack)/change the fluids to use a different container(example: with bob´s mods petroleum gas uses a gas bottle)

I have not tested yet. But if other mod only alter the stack size (from what I read from ReStack code), it should work fine.

If other mod uses a new container, then it is probably a new item (unless Bob cloned the properties from vanilla barrels). Since this mod changes items indexed by the name of the vanilla oil barrels, I guess fuel settings would not be applied to them.
It would need a few lines of code added to set the fuel values properly, but I have totally no knowledge of Bob's settings on oils (like names of new containers, recipe ingredient/product ratios, etc).

Hello, the stack size of the barrels isn´t my concern, but does your mod change the fuel value of the barrels, when for example ReStack changes, how much fluids can be stored in one barrel(for example, when a barrel can store 500 instead of 50 units of fluid)

No, it would be extremely complicated to automatically determine fuel value of barrels, which is especially non-trivial if any barrels are involved in multiple recipes. Automatic code to be compatible with all mods, is almost an impossible task.

The better way would be when "fill-barrel-" recipes are being modified by ReStack, it also detects and modifies the fuel value by multiplier used there.

So, can you make your mod compatible only with ReStack?

As my last reply says, it is much better to make the changes in ReStack, not here.

If you add fuel values to the fluids I will calculate the value per barrel when setting fill recipes.
It makes no sense for only barrels to have a fuel value. Any barrel touched by ReStack will have the fuel value set to fluid fuel value * barrel capacity.

Thanks.
Just uploaded 0.17.1 which included fuel settings to the four oil fluids.
Tested with ReStack 0.5.8 can calculate the correct values.