I own a 98 Malibu with the same engine (3100 3.1L) and the owners manual says 4.5 quarts. (4.5 qts.) THAT IS CORRECT!!! I just checked the dealer service manual yesterday.
It depends what vehicle/engine you mean, but a 4.2 is likely a mid-sized v6, so probably around 6 litres (i think a quart is .95 L. In any case, dont be afraid to buy more than you need to, you can...