Selina is just a brand endorsed by Alfa at the time.
There are far better oils for the same cost out there.
Oil's are a bit of a minefield. But you really need to let us know what sort of driving and what budget you want to spend.
Opie oils / Oil man would be the best person to advise really.
But generally a 5w40 synth that's of a decent brand and got the correct specification and approvals will do the job.
If I ran a turbo car I'd change oil ever 8-10k max. Depending on cost I might even do the oil and filter @ every 6k.
Turbo's put a lot of strain on the oil vs N/A.
Personally I've been using 10w40 Semi Synth in the Twinnie will no issues (Castrol Edge but I'd still say this was a budget oil and even then I changed at 10k). But I'm running lpg and motorway journeys means the oil does get an easy life. Also make sure you get a decent oil filter like a mann or mahle.
Then just buy the correct spec decent oil you can afford.
Millers is wonderful stuff but it does tend to be £50-70 for 4-5 litres.
I've also used Fusch in the past and that appears to be half decent too.
But advise from Damien is sound. Ask Opie oils.
They have the time and knowledge to best advise you.
They also have decently priced gear too.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts