Opel changed the supplier of the bearings roundabout 2009, the new ones where much cheaper for Opel, but much worse for the customer.
HAS-Antriebstechnik, the lead german expert for M32 reconditioning researched that, because they had a lot of trouble with the bearings too - after repairing (changing the bearings too!) and return to their customers the dynamo sound returns.
Since 2015 HAS replaices the bearings with the erlier ones (till 2009) and now all complaints are gone.
And yes, a filling amount of 1,8 litres is also debateble, some guys here put in 2,1litre, if i remember correctly.