It looks like you're referring to a type of motor oil used in passenger cars. "PCMO" stands for Passenger Car Motor Oil, and "5W40" is a viscosity rating that indicates the oil's flow characteristics at different temperatures. The "5" in 5W40 indicates its viscosity at low temperatures (cold start), and the "40" indicates its viscosity at high temperatures (normal operating conditions).
"API SN" refers to the American Petroleum Institute (API) service category for motor oils. API SN is a specification that indicates the oil meets certain performance requirements for modern gasoline engines.
"REXTON SYNTH" appears to be the brand and product name for the motor oil you mentioned. It seems to be a synthetic oil designed to meet the API SN specifications and provide the specified viscosity performance (5W40) for passenger car engines.