I paid something like £10.40 per litre from s4p and with 10% off thats pretty cheap. It also bulks up the order to get free delivery. I have to say in years gone by I just stuck to the specifications. I used mobil 1 for years and never had problems even towing a heavy 5 birth caravan at speed over big distances. I only worry about this thing because TA scares me and my wallet. The GEM tech advisor suggested todays oil additive packages are so specific it matters. Im not sure, but Im old and now cautious.
Yes matters,
Understanding European Oil Standards: ACEA Specifications - Valvoline™ Global Europe - EN
Each vehicle manufacturer recommends using precisely determined oil formulas to provide the engine with optimal lubrication and protection. That is exactly why European lubricant manufacturers need to abide by the engine oil standards set by ACEA. In this article, we explain what ACEA...

As long as it meets or exceeds the specification in the handbook it doesn't have to be the exact oil recommended in the handbook, but you can't just shove anything in
As far as I can see what the original poster was wrongly informed
Fiat have done a callout and we have to use 0W-20 oil in all of their engines,
The only technical service bulletin I can find regarding a change in oil is for the 1.5 diesel engine
Some of the older engines are not compatible with the newer euro 6 oils
The bit after the FCA 955535 number is different for TA, TA turbo and diesel