I've dug out my old Haynes manual for the 131.
They state 4 litres, for all the 131 engines, both ohv and twin cam, including the filter. That seems to ring a bell, and is a good place to start. Add that much, let it settle, use the dipstick.
Haynes manual number is 310, if you can find one.
The Argenta, and I had one of those later, used a larger sump. I think in the 132, there were occasional issues with the sump hitting things, so for the Argenta, they made a wider flatter sump, like an inverted T. That needed more oil. The Argenta was also a pain, as the wide sump allowed the oil to all sit one side when parked on a sideways slope. It also had a dipstick with a level sensor, so if parked to the kerb on the left, on a steep camber, when started, the oil level light would flash. This would continue until next restart, however long. So drive a short distance, stop on the flat, stop engine. Count to ten, restart, continue. Disconnecting the wiring plug caused the light to flash every time, as did connecting the wires together, so I never did find a way to disable it.