2004 VW Jetta GLS: always using full synthetic oil (Castro Edge 5w30) how often should the oil be changed? Can't find my manual or info online.
It depends on which engine you have, but following this guideline, you should do as follows:
Engine Frequency TDI First @ 5k miles, then @ 10k, then every 10k miles 1.8T Every 5k miles 2.0L First @ 5k miles, then @ 10k, then every 10k miles
NOTE: I don't actually believe the 2.0L came in the 2004, just the TDI or 1.8T, but I could be wrong.
Hope this helps.
It also depends on if your car is set for long life service intervals or not. I've got the Bora 1.9 TDI PD130 (2004 model year) and it seems to need oil changes every 20k miles.
Checking the oil level regularly is very important as low oil on these engines will cause untold damage. Also, if you change the oil more frequently than required then this is going to help, not hurt your engine.