Dunno whether you've sorted yourself out now, but I had the same
question, so was prompted to e-mail my Alfa dealer. Their reply was :

We use Selenia 20k oil which is rated at SAE 10w 40 and is a semi
synthetic oil. Selenia is the approved brand of Fiat and Alfa Romeo
but
you could use a different make like Shell or Castrol as long as it is
of
the same specification. We do sell Selenia in two litre tins for
£12.51
inc VAT.

This is Cleveley Cars in Cheltenham.

Cheers, Keri.

147 2.0TS