If the previous sludge issue did not need a tear-down and the turbo lines are clear, just use any Group IV oil that meets the 502 spec. required and diesel filter. It looks like this is what you have been doing, so don't fret and steady-as you go.
Keep good records, because as you may know, VW has extended the engine warranty (due to sludge) to 7 years/unlimited miles. There is no need to risk clogging up the oil pump with non-approved oils, only to nix any future oil sludge warranty claim.
You could save some money by buying a case of filters from an on-line OEM supplier.
How is the timing belt & tensioner?