Connect the inserter to a chest with a red wire.
Set the circuit condition to oil barrel < 10.

Then you need to slow down the chest unloading to keep 10 barrels in the chest until the train leaves.
I didn't get the second part of your post. So that means the Inserter won't work after ten oil ...

I'll just keep it simple. A train full of crude oil barrels stops at an outpost and starts unloading barrels. It unloads the whole lot. I want only 5 or ten (let's say 10 for convenience) barrels to be dropped off. How do I tell the inserter to take only ten out and not one hundred.

I think I put my oil low at the beginning of the game
It is scarce. But I have one wagon with half dedicated to full barrels and half empty.

Station A is my main area where full barrels will be unloaded. Station B on the same line has a small production for plastic. It has its own pumpjacks but I ...

Hello all, I don't know if this is covered but I've made several oil outposts and the train stops at each outpost and gives out empty barrels in return for the full one. They thing is I want every oil outpost to give five empty barrels out because it doesn't need more than that.

My setup is like ...