It is my understanding that the "service soon" light is based on both time and miles, if it was not reset the last time you had your oil changed it may have just "timed out" and needs to be reset. Most manafacturers have a sequence to go thru with the ingnition or other reset keys that will reset the clock/miles in the ECM and away you go, you should be able to do this sequence yourself. I would not worry much about that light being on (unlike the check engine light) since i do not beleieve that in 2010 the E-450 had onboard dignostics that actaully evaluated the condiiton of the oil in real time and send a meesage when the oil is actually reaching the end of its life the way 2013 cars currently operate.... I may be wrong
On my 2010 E450 (same as OPs), the "Service Engine Soon" symbol is not used to indicate that normal maintenance servicing is required such as an oil change. According to the manual, the "Service Engine Soon" symbol on the 2010 E450 indicates the On Board Diagnostics System (OBD II) has detected a malfunction. This would require that the OBD II system be checked for codes to determine what the malfunction is. There are hundreds of codes, some are government mandated and some are vehicle manufacturer specific, and the malfunction can be a small issue or very serious. So the "Service Engine Soon" symbol is in fact a check engine light and should be investigated soon.
My other vehicles have those systems that indicate oil change required after time, miles, or monitoring of engine load. Maybe the later E350/450s have some sort of maintenance reminder indicator in addition to the OBDII malfunction indicator?