09 Passat 2.0L alternator and ground points

Hello

I was experiencing low charging voltage (13V) and battery voltage when off at 12.0V. I replaced the original Valeo alternator with a Duralast Gold alternator and replaced a 1yr battery (under warranty) with a new battery Duralast Gold battery. I am uncertain if the problem still persist as a I see charging voltages down to 12.6V and then goes up to about 13.4V. I noticed that the voltage difference between alternator casing and battery negative terminal moves from .3V to .6V. I have done some cleaning of ground points but dealer tells me that I have to install a Valeo alternator.

My questions:
a. Do I need a Valeo alternator? I have seen cars (Fords) who do not operate well unless its the OEM alternator and therefore my question on VW.
b. Is the .3 to .6V voltage at alternator casing normal?