Synthetic Oil for Towing?

I have been putting rotella T6 syn (diesel ) in my engine since 6k miles. I get it for $19.00 a gallon at any Meijers, some time wally world will have it for the same price, and at 10 qts it is not a expensive task as most people think. I also change it at 6k miles. With 99k miles and heavy towing, it gets a sample sent off and everything is perfect