Changing the oil? Not sure what you mean. Oil changes are based upon time/mileage. One adds oil periodically to top off as the oil does eventually "escape" by one natural means or another not so natural means.
To your other question, I do not have the books accessible to me at this moment, however I did a quick-n-dirty search, DFINNEGAN posted on Rennlist: "The stick reads 1.75 liters top to bottom and the gauge reads 1.25 liters (+0.3 to -0.2 liters)." That may explain the difference we observe. The gauge being a measure of the reservoir level which being a reservoir can be +/-. That would be in reference to full oil which I believe is something like 12 or so quarts (yep gallons!).
Note that Rennlist is analogous to Pelican Parts. Very knowledgable people. Equally nice too! Only difference is lots of Brits on Rennlist so if you start making Benny Hill or Monty Python references they get it. Here, not so much. Note: never make reference to the Monty Python "Bruce skit", there are Aussies on that forum too (as well as Pelican), they may not have the same humor as Brits
In the end, if you are concerned about how much oil is in there, the above gives you an idea. Between the dipstick and the oil gauge one can keep things within limits. Additionally one can also ascertain whether their oil gauge is "reasonable". If it isn't you can then proceed to narrow down if it is the gauge, wiring, or sensor.
Enjoy it!!!