You can find these informations in your handbook. If you don't have it (or like me, don't want to go to your car when you are behind your PC ) you can dowload it at : http://aftersales.fiat.com/elum/
For your car (same engine as mine), engine oil is 5W40 ACEA C3
SAE 5W-40 ACEA C3 synthetic oil to FIAT 9.55535-S2 certification. (PETROL)
SAE 5W-30 synthetic oil. to FIAT 9.55535-S1 certification (DIESEL)
For diesel engines, in the event of an emergency in which the original products are not available, lubricants with at least ACEA C2 performance are acceptable; however, in this case optimum engine performances are not guaranteed and the lubricants should be replaced with recommended products as soon as possible at a Fiat Dealership.
The use of products with specifications lower than ACEA C3 and ACEA C2 could cause engine damage not covered under warranty
Since its a bank holiday your probably stuck with halfords
Look for Castrol edge fst. It says diesel on the bottle but matches fiat spec