Oil change, what a difference

Normally I change the oil in my vehicles around 3k miles. Most past vehicles I'd notice slightly better gas millage, sometime a little bit smoother running engine. I just bought my Z, changed the oil before I started it up the first time. Ran good. It's been about 2,000 miles and the oil was looking very black for the miles so I decided to change it again. I changed it and test drove it; it make a pretty big difference driving it as far as power is concerned. I actually felt like a 5hp difference. Maybe the person before me didn't take care of it and it had quite a bit of buildup I removed? Maybe the design of the Z's engine just causes this condition? Anyone have any input of the subject?