Peugeot recommend the same oil in both their petrol and diesel engines: Total Quartz Ineo ECS 5W30 -
http://www.peugeot.com.au/media/deli...305-df583b.pdf
They make the diesel engine in the Focus so maybe that would be the best oil to use.
The engine is the DW10BTED4 and depending on your Focus' year model it may or may not have a Diesel Particulate Filter (DPF) or FAP.