Hi,
in IS_PIT there is a byte called FuelAdd
but it actually displays requested tank fill percent which doesn't always match the actual amount filled.

Is there a way to get the correct amount of fuel added via insim?

There is Fuel200 byte in IS_SPX and IS_LAP which displays approximate amount of fuel left in tank when crossing a split/start/finish.

So adding Fuel200 to the IS_PIT or adjusting FuelAdd would be enough?
Or maybe a possibility to request fuel amount with IS_TINY or adding it to CompCar in MCI could also be a possible solution.

In what case is it not correct, when the requested tank fill percentage + current fuel percentage exceeds 100%? Or are there other cases?

I'd say, if the approx percentage fuel left + requested fill percentage exceeds 100% you could cap it to calculate the actual percentage of fuel added.

Problem is the current fuel level is not known, only from the last SPX or LAP packet, so custom InSim laps (or just layouts with no laps) don't ever get it.

There is one spare byte in MCI packets which could be perfect for this, and not require extra data transfer.

It would be so cool if we could set the vehicle fuel from InSim