Aye, I to run delo 400 in my Jeep. Our others equipment, 2000 sebring convert., 2003 trailblazer, lawnmower, power washer, all run Rotella T 5w-40 year round.
For winter though I switch over to Rotella T 5w-40 in the Jeep, I can get the rotella and delo at Wally-world so it works out great. But I know the Jeep's older 4.0 really appreciates the 5w oil in the winter as is evident by the lack of startup noise when cold.
Switching between the syn and non-syn doesn't seem to have any affect at all to any of the vehicles.
My reason is that the Jeep is used mainly for trail riding in the summer in 4lo. The highest rpm's it sees there are in the range of 1500. So I'm worried about the wide range 5w-40 being more prone to shearing than the 15w40 due to the heat, and low rpm/high torque conditions. There i go rambling off again...