The oil service indicator does not actually relate to the SES light. Your SES light comes on when there is a fault relating to emmissions. Most auto part stores will scan this code for you for free and offer advice on how to correct it. The SES light can be on for many, many reasons so getting this code is the key to helping to narrow this down.
The code should be in P code format (p and 4 digit number). The code is stored in your DME control unit and can only be retrieved with a scanner.
If you can find out the code I might be able to help you again with some general information on what the code might mean.
I hope this is helpful. Please accept and let me know if I can help you further. Thank you.