Well, are you asking about the M.I.L (Malfunction Indicator lamp) aka the check engine light, or are you asking about the engine oil replacement reminder light (aka Maint Reqd light)?
they are shown in the US 2004 owner's manual, pages 5, 117-118, 120-123.
If you're asking about the Maint Reqd light, it isn't connected to any engine or oil analysis - it just starts blinking 4500 miles since reset, and stays on solid 5000 miles since last reset. It's only a reminder to go in for a service, nothing more.
The MIL, however, tells you that there has been some error code (DTC) in the system, and if it is on you may not pass an emissions inspection... It'll stay lit so long as there is a stored error code, and it'll keep adding the codes to its list of stored codes... Some codes will clear themselves after a certain number of restarts where the problem doesn't occur again (like the loose gas cap code), but most others requires the dealer's scan tool to clear those codes.
To clear the Maint Reqd light (if you do your own oil changes, or if your dealer/service tech forgets to do it for you):
Page 122-123 of the US 2004 Prius Owner's Manual (Section 2-5 "Operation
of Instruments and Controls: Gauges, Meters and Service Reminder
Indicators", item (j) in the "Service reminder indicators and warning
buzzers" section.
http://autos.groups.yahoo.com/group/Pri ... aintenance
http://www.vfaq.net/FAQ-screen.html#maintenance
page 14 of the User-Guide:
http://john1701a.com/prius/prius-userguide.htm