for 1.8 you need 5w40 (preferable alfa romeo selenia oil) lower viscosity will also be ok (but could result in more wear) higher viscosity is a NO NO
1.8 takes a total of 4.2 litters (this includes oil filter capacity)
to top up - leave standing overnight on level surface - in the morning withdraw dip stick and check level (the distance between 2 marks is 1 litre) if you've half a litre low - add 350 mil and check next morning (adding oil faster is not practical on 1.8 as it takes eons to trickle down)
I like 5w-40 fully synth in the JTD. Is this an extravagance? Will 10w-40 part synth do? I do quite a few motorway miles (13000 private miles since December) and change the oil after 8k to 10k.