The first oil barrel placed in the excavator (which gets consumed immediately to fill the oil buffer) does not output an empty barrel - subsequent ones do though

I play with k2so & barreling group mod, if that matters

Barrel gets returned after you used all fuel

Its burn result.

Ah, but in that case manually picking up the excavators while they have fuel should give you the barrel back (which isn't the case). And that's something you do a lot, cause you constantly replace them over new ground, causing you to loose barrels

that's true, but that's result of how game itself work, and I am not sure I can do a lot about it.

I think the most natural approach would be to not use burn result, but to make the excavator perform unbarreling recipe to fill some small internal liquid buffer (of a size of 1.5 barrel's contents?) and so immediately return the empty barrel

I think it's easier and less messy to just use exising game mechanic.