1998 Toyota 4Runner engine smoke only when oil is low

Symptom: visible blue smoke came out of tail pipe; checked oil level (very low); added oil (added 1 qt 5W-30 along with Motor Honey); and no more smoke (at least for now) 02/28/2009.
Background: I brought this 1998 4Runner on 02/20/2009. It passed emission without any problem. No leaks on the ground. I changed the engine oil and added gas treatment. 185 miles later, the above symptom occurred. Any idea what can be wrong? Any help would be highly appreciated.