Hi Brymak,
Good to hear that you're enjoying the Uno experience
Other pluses that I can recall are: excellent space, excellent visibility, less chance of expensive damage around town and also cheap to fix!
Personally I wouldn't use a lead additive, even in engines pre-88 (supposedly, well according to a Haynes manual anyway, engines after 1988 can be used with unleaded fuel). I grew up with our FIAT 128 and that ran on unleaded fuel for years, and I've never put an additive into any FIAT I've owned, and never had a problem.
Of course it is your decision. Monty's Strada/Ritmo Abarth 130 is a rare breed, whereas Uno engines are commonplace if something bad happened. But most Uno engines in NZ seem to die from broken cambelts and blown head gaskets (I have fixed both of these faults in cars I've bought!) so I've taken off a handful of heads from indifferently-maintained Unos, and haven't seen any valve problems.
As for the octane, I always remember what Maurice Thomson told me years ago when I was having starting/plug fouling difficulties with my 128 Coupe. He asked me what petrol I was using, and (in 1997) I was using the 96 octane. He said to put 91 in, at least it was consistent, whereas 96 was "all over the place" in terms of octane number (back then, don't know about today). I have a friend who has had plug fouling trouble with the 98 that you now get at Mobil. I have the feeling that the 91 is the cleanest-burning of the fuel that we have, since it has less benzene and other octane-boosting crap put in. This is from watching the exhaust pipes of various cars I've run.
Sooo... run on 96, and if you know that the ignition timing, spark plugs etc. are all in top-notch condition, try putting 91 in. Listen for a tinkling noise during acceleration, particularly in (say) third gear going up hills. The tinkling (pre-ignition) is bad for the engine if it happens all the time, but if you can drive gently (and economically) without tinkling, then 91 is OK for you. I think with the Uno 60, though, you will conclude that 96 is better (I did, at least).
Other 91-problems to watch for are: jerkiness at low speeds, engine runs-on after switching off (less likely with yours due to the cut-off solenoid on the carb). Other 91-benefits are: possibly easier starting and the small cost saving.
Don't bother putting 98 in unless you're keen to play with the ignition timing (more advanced) and want to see if you can extract slightly more performance than with 96. Probably not worth the extra 5c/litre.
Just my ideas...
-Alex