Synthetic is exactly as it sounds....it's not REAL oil, drilled from the earth. It is an oil created in a lab. Synthetic oils have more detergants in them, and last longer than their conventional oil counterparts. While oil companies want you to change your oil every 3,000 miles. It has been proven time and time again that even conventional oil in a healthy motor can go 5,000 and even 7,500 miles per oil change. Synthetic oil can last over 10,000.
OCI = Oil Change Intervals.
Ryan