Hi Guys and Gals. First off this is a great forum and whoever put it together did a top notch job. I just got my GMC 2500HD D/A two weeks ago and need to change the oil for the first time. I have always been a Pennzoil man in my previous Gas engines. I see that Pennzoil has a 15/40W Heavy Duty oil with the required CI-4 rating. Any comments, good or bad, about this brand of oil?