In the attached quote I have quoted you three alternators.
The 3884950 is the Volvo Penta OEM version.
The 18-5882 is the Sierra. I only found one or two references for this being a replacement for the 3884950 and Sierra actually does not include this number as one of them. However they do include the earlier number for this alternator.
If you want to go aftermarket I would go with the Arco 60073 alternator which is a direct replacement and less expensive. However, these aftermarket ones are not quite as strongly built, but they work fine.
Volvo Penta would be horrified to hear that you are thinking of using a non-volvo synthetic oil, but I think as long as yours meets the specifications and you don't go really cheap then you would probably be OK.
The ZF63 does use the Dextron III AFT. I quoted you the Volvo Penta ZF filter which is, for some odd reason, less expensive than the ZF version. We have both.