I have driven mine with gear oil and with ATF and prefer the ATF (because I have had two different trannys in the car, with the original one back in now). You don't get the hard shift when cold issue, with ATF. The gear oil needs a little warmup time to start shifting nicely especially in the winter but to some degree even on a summer morning. The ATF just makes shifts a bit lighter in action, gear oil a bit heavier in action, no surprise there, really. But use whatever makes you feel most comfortable using.
Charles
Thanks for that - yes, the one thing I hate most is the first few 'crunches' in my Uno Turbo, which has a similar gear oil recommended. The oil that I settled on for that was a 75W gear oil plus the Nulon additive.
I think in the 164 I'm going to try a 'base' of ATF plus the Nulon additive, which I have found to be pretty magic - it's basically a white goo that seems to improve friction for synchromesh and at the same time quieten bearings. It comes in a white-coloured tube clearly labelled 'manual transmissions additive'.
The one thing I don't like is the EP additives in gear oil - not necessary for our FWD transmissions, and back in the days of the 128, they brought undesired effects (eating into phosphor-bronze synchroniser parts). Obviously the newer gearboxes (such as that in the 164) are able to use EP oils satisfactorily, but why bother when there are better additives available (e.g. the Nulon additive I mentioned).
I've just realised that Nulon is Australian (easily available here and well-priced), and probably not known in the US. here's the page.
http://www.nulon.co.nz/products.php?productId=g70
It makes a very pertinent comment: "...Many modern gearboxes suffer from poor gear shifting when the gearbox is cold. As a result a number of manufacturers are recommending a lighter grade of oil, and in some cases automatic transmission fluid. These lighter grade oils provide superior gear changing at the expense of reduced life of gears and bearings, because the lower viscosity oil does not have the degree of EP (extreme pressure) protection. Nulon G70, due to its EP characteristics, will greatly improve protection as well as dramatically improve gear changes."
Since I've had success with this additive for several years now, I'm inclined to believe this comment, but the idea of PTFE going into the gearbox is a little concerning, I have to admit. You would have to wonder if the synchromesh efficacy would reduce with all the slippery stuff in there, but I have no negative experiences. Do note also that this additive doesn't have the 'EP smell'.
Alfisto Steve: your bearing replacement experience rings true with me - I've had to change input shaft bearings on three or four FIAT gearboxes now, and I suspect the original bearing (for the C503, a one-off strange size from a French manufacturer, if I remember correctly) is just not as good quality as it should be. It probably doesn't help that it's the smallest diameter bearing in the 'box. So even with the 'thick' gear oil, that bearing still fails! Gives what I call the 'old bus' noise when moving off, most noticeable in 1st and 2nd.
EDIT: Just realised your distances are in miles, 209,000km is a good innings!
Now, how did you size-up a roller bearing replacement? I think it's a combination of imperial-sized ID and metric-sized OD, isn't it? My bearing suppliers were stumped and had to order one from France, identical to original (I judged that another 180,000km would be enough for that FIAT Croma). At about $200 (US$138) it was several times the price of the usual metric bearings used in the smaller FIAT transmissions. In fact the owner of the $500 (US$345) Croma nearly had a heart attack, particularly with my $150 ($104) labour charge on top (not unreasonable I felt, as it included the clutch replacement and a diff swap to fix the broken speedometer drive gear). I did have to take the gearbox apart after all!

Interesting how a part becomes a lot more expensive when it's bought for someone else.
Cheers,
-Alex