got my first oil change and gas milage took a nose drive

[timcrow00]

man i wish you guys could just answer a question instead of picking it apart! i try to keep things short and not write you guys a paragraph that takes forever to read. when i bought my truck it came with free oil changes(basic oil only, synthitic is about $30 extra). so at 5000 mi i took it in for the first oil change. after i got the truck home i checked the oil and it was already dirty. i guessing the guy who changed it didnt let it drain very long. so i went ahead and put mobile 1 in it and now the oil is nice and clean but my milage and performance is reduced. i know because i used to be able to drive to my hunting area and back on 1/4 of a tank but now it takes atleast 3/8 to do the same and i can also feel the power difference is reduced. i know this is not firm evidence of a change but i know my truck and im not stupid.