T-300 Cat 3126 ????

I just purchased a T-300 2003 295k miles, from a well know trucking co. They assured me it ran well. But in driving it home, top speed was about 55 was all I could get out of it and going up the slightest hill, I had to shift down. They said they changed oil every 15k miles and it was due. I have heard from others that you ought to change every 5k or they will run poor and loose power. I just got it home and haven't had time to change oil. I did change fuel filters on my way home and checked the air cleaner. Didn't help. I have a 2002 FL 70 with the same engine and more miles and it runs fine. I change every 5k. If I have to take it to a dealer is it better to take it to Cat or Kenworth?

 

I am not a fan of the 3126 but I have one in a Fl70 and one in a GMC 6500 so I know what they should do. I am sure it is not a governor speed because it doesn't have enough power to hardly pull itself and going up the slightest hill or should I say incline, you have to shift down.