I believe that 4/4 TC & +4 both use engines from the Fiat Argenta.
In my Argenta Owner Handbook it say 1 kg (1litre) of 15/40 between min & max marks for all petrol models.
For the 2 litre the Haynes manual says 4.83 litres inc filter.

Jack
1985 4/4 T/C 4 seater Royal Ivory
1934 F Type
2018 Pembleton SuperSport (self built).