Need help!

I put Mobil 1 fully synthetic oil into my impreza for the first time( I used conventional oil before). Anyway, I took it for a test drive and the engine has no power what so ever. Everytime I go into the higher gear the engine sufficates and runs very loud. I did not do anything else besides changing the oil filter and oil; it was running perfect before the oil change, now I am affraid to drive it so I do not damage the engine. Has anyone had something similar happen to him? Any suggestions what might be the cause of this wierd behavior? Thanks

Did you overfill or underfill the engine with oil?

According to the Subaru owners manual the capacity is 4.2 qt. First I put 4 qt. started the engine until it reached normal temp. then I turned off the engine checked the oil level and added about 1/2 pint. After I did this I checked the oil level and it was within range. I though that putting to much oil could potentially cause my engine to run so rough. So, today in the morning I drained the oil and refilled it again with only 3.75 qt since I did not change the oil filter I figured probably there is around 0.25 qt in it. I guess I should check the spark plugs to see if I have any oil on them which would mean I have blown a gasket; however, I see no leaks anywhere. Now that I think of it I also changed the air filter. Maybe the air filter is causing all this mess, but if it is this would be the first time I had something like this happen due to the air filter. I also thought maybe I should flush the engine and try again, I may do that. although Mobil said no flushing is required. I don't know.