Hello & welcome to the forum. You are correct in assuming that you have a Mark 1 C5, however there is confusion over Mk1s; Mk2s and Mk3s. In 2004 Citroen never called the newer vehicle a Mark 2. Only the "facelift model". So it's better to refer to the car by its build date. Only on the 2.2 engines was the particle filter fitted from 2001. The 2ltr engines didn't get them until mid-2004. Engines with particle filters benefit from using a low ash oil.