You need from VW:
035 133 557 - injector insert seal - 0.87 x4
and from GSF:
18720 SEAL-RING-INJECTOR G2 16V 2/86 >10/91 0.60 x4
Or from VW if you don't want GSF ones:
035 133 557 A - injector seal - 1.88+VATx4
You use merc injectors because they are cheaper, and made of brass so don't rust. Theres a guy selling em for £50 a set on ebay ATM
