In the Castrol there is the SLX and The SLX II.
They are different as they cover certain VW specs but not the same ones.
SLX = VW 502.00, 505.00, 503.01
SLXII = VW 503.00, 506.00, 506.01
The specs are weighted between petrol and diesel.
SLX
502.00 = Petrol
505.00 = Diesel
503.01 = Petrol
SLXII
503.00 = Petrol
506.00 = Diesel
506.01 = Diesel
Castrol is expensive and you may find Fuchs better value for money.
Their equivalents are:
Titan Supersyn SL 0w-30
502.00, 505.00, 503.01
Titan Supersyn SL LL 0w-30
503.00, 506.00, 506.01
Fuchs are also VAG's Factory fill partner.
Hope this helps,
Cheers
Simon