Petrol or Diesel on the side of a can means exactly nothing. It's the specs that the oil meet which decide if the oil is suitable for your car or not. For example, the Castrol Edge 5w-30 meets the latest BMW, Merc, VW, ACEA etc diesel specs, but no mention of Diesel in the name. Mobil 1 0w-40 Turbo Diesel meets some diesel specs, but not the latest ones. All oils are suitable for petrol or diesel engines in general, but there are the modern diesels with diesel particulate filters that need the right low ash oil, so not all oils can be used in those. Having diesel in the name doesn't mean that the oil is any more suitable for those cars than the next oil though. It's all down to the specs, not the name.
Does this mean I could use Castrol Edge Fst designed for petrol engines instead of the turbo diesel one? The handbook for my 2.4 jtdm with dpf states it needs synthetic 5w-40 meeting Fiat 9.55535-n2.
The specs for the castrol oil are
SAE 5W-40, ACEA A3/B3, A3/B4, C3, API SN/CF, VW 502 00/ 505 00, BMW Longlife-04, MB-Approval 229.31/ 229.51, Porsche A40, Renault RN0700/ RN0710, Meets Fiat 9.55535-S2, Dexos2
SAE 5W-40; ACEA C3; API SM/CFp, VW 502 00/ 505 00/ 505 01, BMW Longlife-04, MB-Approval 229.31/ 229.51, Renault RN0700/0710, Meets Ford WSS -M2C917-A, Meets Fiat 9.55535-S2.
I take it that means I could?