Bad Oil
 

Is it possible that an engine full of dirty old oil can reduce fuel efficiency and cause overheating.

I only ask because about 2 weeks ago I did a complete oil change and since that time I've noticed that the fuel gauge is taking much longer to go down than usual and the car hasn't overheated once, which is unusual.

I could see fuel efficiency going up, but for overheating...only if ther was sludge built up in everything.