I have a 2002 ford explorer sport trac (Just bought it) with 56000 miles I switched to Castrol Synthetic The first 3000 miles I had it I got about 350±25 per tank (22 gallons)since I switched I can get barely 300 any ideas?
At 56,000 miles, you could perform the scheduled maintenance for peak performance, which would maximize fuel mileage.
I wouldn’t blame the oil. Many factors control mileage. Driving habits, conditions, ambient temperatures, gasoline quality, etc. A raise or drop of 2 mpg (50 miles / 22 gals) is normal with the switch between summer blend and winter blend of gasoline.
Switching to synthetic oil noramlly would IMPROVE your gas mileage. So, don’t blame the oil. A change in driving style and pattern can also affect mileage. Agree with other posters that doing required maintenance, such as new air filter, spark plugs can greatly improve your mileage.
What weight oil were you using before switching and what weight oil after? If you don’t know what oil was in there when you bought it and that was the first oil change, it could well be that the prior owner had a very light oil in there for some reason.
Note: Measuring mileage as miles per tank is seldom very accurate. To check accurately you need to measure using miles and gallons (which is why the result is mpg miles per gallon). Do this three times at consecutive fill ups. remember to check the mileage at your next fill up. and mark it down. At the next fill up mark down the number of gallons to refill the tank and the new miles. Divide the difference in miles by the number of gallons, and repeat.
Agree that NO ONE should post a gas mileage question unless they had run 3 tanks of gas from the same station, same pump, same driving pattern! In my own car I have mileage ranging from 41 mpg (highway, no A/C) down to 29 (all city driving). Different gas pumps will shut off at vastly different times. We do not advocate trying to get the last little bit in for danger of damaging the vapor recovery system.
Switch back and see what happens. Save the oil in a container in case you want to put it back in later. Did you use the same viscosity oil?
Per tank is woefully inaccurate. Fuel guauges are never linear and you may have had more time(miles) available at the point when filled the tank and the needle would never move.
The only way to tell MPG is divide miles travelled by gallons consumed. In other method is pure guess.
Thanks everyone. I did do some research before I posted this and a little more after. Of course, I found my answers after I posted. For the record though… I used 5W 30 and I also used the same pump and Octane over 3 tanks before I posted because I know driving habits and all that can change the output. However this last tank I tried to drive really conservatively and got the worst performance yet, I’m pretty sure it is just due to it being winter. I also moved from FL to VA and didn’t even consider the climate change until after I posted. Thanks again! 
I’m going to go sulk in the corner now.
I suspect something got pushed or disconnected at the time of the oil change. Something such as a vacuum hose. Look for a disconnected hose.