"Check Engine" light? Or change oil light?

You said Check Indicator. I understand Siennas have an oil change reminder every 5000 miles. You can turn it off after you do your oil change.

If it is a check engine light..there are lot of posts about that. Most likely cuases are --bad O2 sensor, gas cape not tight...alot of possibilities.
You don't need to bring it to a Toyota dealer for service if it is out of warranty.
If it is the check engine light, the easiest way for me is: Go buy an OBD reader from autozone($40-$60) and check the code, and then erase the code. Drive it for a week--it is not going to damage your car, unless you already smell unburned gas, or see white/black smoke from your gail pipe. If it comes back on again--replace the front o2 sensor, then drive it again...
If you bring it to a garage, they will quote you the cost of replacing all the o2 sensors (2-3). Total cost cuold be $500-$800...
Good luck!