Hi Linda330,
Welcome. May I suggest that you consider this logically.
1. How many miles has PVC covered? I have not checked the handbook, but I do not think that Fiat make any recommendation for changing the gearbox oil.
2. I do not know what "condesation problems" your garage is alluding to, but if the oil has been contaminated, the vehicle would benefit from a change.
3. If changing the oil, there will be an opportunity to inspect the old oil for contamination, particularly metallic debris., indicating worn components.
4. There has been some discussion about not using oils to specification GL5 which has a higher additive content, and cause problems with the synchromesh components. The lower additive specification GL4, is now more favoured.
5. Draining the oil out is easy enough, but refilling could be more tricky. The official line is to remove a breather valve on top of the gearbox. Can you see the top of the gearbox from under the bonnet? Access with the vehicle on a lift may help, but garage may also need a filling pump. A DIY solution is to use a funnel and pipe, but the pipe has to be threaded into position.
6. Perhaps your garage does not wnt to get involved, when there is no guarantee of any improvement?
You may find
this thread interesting.