Following on from John's point, the first issue that garages come across is the need to drop the lower engine cover before they can get a sump spanner on the change the oil. This is time consuming. So the practice of sucking the old oil out through the dipstick is getting more and more common. Unfortunately this doesn't guarantee that all the old oil and the sludge deposits are removed.
This isn't good practice for the 1.6 (or any other engine for that matter), but the 1.6 seems particularily sensitive to this type of procedure. PSA have instructed all their dealerships on the correct servicing procedures for Citroen and Peugeot vehicles which I presume will also be best practise with Fiat dealerships. This is that the oil must be drained when up to operating temperature through the sump plug, the vehicle must be level, and a minimum dripping time (20 minutes I think). Oil filters must always be changed and servicing must be done annually if mileage is low.
Diesel fuel filters should also be changed every year depending on mileage, but in practice this can be every 2 years if you are doing low mileage.
The old shape Expert/Dispatch/Scudo that ceased production around 2006 only had the 2.0 hdi engine and a payload of 900kgs. Putting a 650 litre tank in this van is tight as overloading is a inevitable, especially with a passenger.
With the launch of the new shape came the 1.6 Hdi offering and the higher payloads, so again choosing the correct payload is important. The 1.6 has a payload of 988kgs. You could get away with this van using a 650 litre tank as a sole cleaner, but would need to be careful with twin operators and addition equipment. The higher payload of 1188kgs has the 2.0hdi engine rather than the smaller 1.6.