I rebuilt my 1990 Audi 80 non-q 4 cyl engine. Now I want to use synthetic oil in it, but I am not sure what grade to run. Castrol treats it as a 12 yr old engine and recommends 30w with no syn listed. Mobil –1 doesn’t know either. My temps here run from 10-100f and I am thinking of using 0-40. Anyone with a suggestion?? Thanks. 1990 Audi 80 2.0L non-q Rathdrum, Id.