I'm about to change my oil in my truck and I was wondering about switching to Shell Rotella T 5W40 Synthetic Oil. I see the stuff at Walmart and it's got GREAT reviews among the heavy-duty guys. Any reason not to run it? The viscosity isn't that different from 5W20/5W30. I did some research online and Chrysler recommends using 5W20 in Hemi engines with MDS. Our PW's don't have MDS so I assume I'm safe to use this oil? I'm out of warranty if that makes it any different.