I have a 2010 Honda Accord EX-L and the check engine light came on. What causes this? I know there are probably a lot.
I took my car to get the oil changed and they put 10W-20 in it. The Honda dealership would put synthetic oil. Does this make
any difference to change it over?

 

A different oil spec would not thrown a Check Engine Light (CEL), unless it were really something wildly different (and even then, I doubt it would). That CEL, though, would have produced an OBD II code (https://www.obd-codes.com/trouble_codes/ ), and that will tell you more or less what the problem is. Auto parts chain stores like Autozone, Advance Auto, etc., will typically check codes for free. You can also pick up a cheap code reader for $25-30.