Honda 2003 Civic Si Owner's Manual page 167

Engine Oil
The oil's viscosity or weight is
provided on the container's label.
Select the oil for your car according
to this chart.
Ambient Temperature
5W-20 oil is formulated for year-
round protection of your Honda, to
improve cold weather starting, and
to help your engine use less fuel.
168
Maintenance
Synthetic Oil
You may use a synthetic motor oil if
it meets the same requirements
given for a conventional motor oil: it
displays the API Certification Seal,
and it is the proper weight as shown
on the chart. When using synthetic
oil, you must follow the oil and filter
change intervals given in the
maintenance schedule.
Additives
Your Honda does not require any oil
additives. Additives may adversely
affect your engine's or transmission's
performance and durability.