The original systems did not have an overflow or vent system, so hard to judge when full, other than the obvious that it wont take any more, which means that its too full. In the mid 90’s a modification was introduced, which fitted a overflow in the reservoir which was connected to a plastic bottle located some where in the lower part of the boat to collect any fluid due too expansion or over filling.
I have a filling point near the outside helm possition with a sight pipe undeneath.
It is always losing oil out of the the filler cap but never needs filling up.
I have had the marina check the system and they conclude nothing wrong !