Checked dipstick today. When wiped clean, reinserted and extracted the dipstick does not show any oil at all. There is not a min/max marking - only a small section near the bottom and just under a inch long where the round metal rod has been flattened and a criss-cross design stamped on it. Are the bottom and top limits of this stamping meant to indicate min and max? Car has only done 18,000 and had full service at around 5/6,000. I want to top it up before next service in about four months. No warning lights have ever illuminated on dashboard. What grade of oil do I buy?