This makes sense because igniting cold air and cold fuel will detonate slower than igniting warm air and warm fuel. I'm no chemist, but I know for sure that for any chemical combustion to happen, a flash point temperature must be reached. So if the temperature of fuel is too low, the spark plugs have to work harder to detonate the fuel. Also, high octane is known for delaying detonation so also take that into consideration during the cold weather that it will most likely change the engine's timing characteristics.

 

The Honda service dapartment overfilled my car with motor oil. I'm thinking that this could be the reason for my drop in mileage.

 

Yes, it could. Depending on how much extra oil was added the hp required to pump air from one cylinderto the next increases as the difference between the oil level and the side of the cylinder wall decreases.
Thats why in economy runs oil level is reduced by half a quart ... ; thety can get away with it because there is lower chances of generating high cornering loads that would cause the oil to flow away from the pump intake and starve the pump for oil.
And no, its not a good idea for the way most of us drive.

Now worse news, if enough extra oil was added there could be contact between the bottoms of pistons and the oil level and that is far more serious as connecting rod and crankshaft bolts can be damaged. If the level was 1/2" above normal you could have that condition.