After a recent advise here, I changed my 80W-90 gearbox oil in the WM20 with engine oil.
The thing is that the box will take only 400 cc instead of the 600 cc as described in the tech pages.
Is there a reasonable explanation for this ?

The gearbox takes 1 pint of oil which is 568cc. As long as the oil is at the top of the filler plug you are OK.