The full name is Castrol SLX Longlife 3, the back of the container holds the important info, it should read VW 504/507 and this is the HTHS (High Tempreature High Shear) longlife oil.
My dealer didn't know it existed, argued with me, told me I was wrong and then telephoned Castrol New Zealand before apologising and thanking me.
I didn't need his humility just the good oil.
Make sure you get the right stuff put in, even if you buy it yourself and give it to them.