page 23 of THIS Garelli manual 1976-78 says use 13.5 oz (400cc) of SAE 30 wt non-detergent motor oil in the clutch housing.. A 1980 bike is likely the same .. except for a top tank model they appear the same to me.
It is wise to use non-detergent ...
Detergents are for loosening up crap and to keep it circulating until it finally gets trapped in a filter. But the bike has no filter. It's better to leave metalic flakes and other junk stuck wherever it is now stuck..
Another thing is detergent additives break down under the heat and pressure of the clutch and glaze or otherwise damage clutch pad surfaces.