I went into Napa the other day and picked up an oil filter. I got talking to the guy behind the counter. I told him I use Royal Purple and change the filter every 5000 miles (oil every 15,000 miles). He explained to me that synthetic oil was "too slick" for the 6.0 and doesn't lubricate the fuel injection pump properly. Then he started talking about how the cab has to be pulled off the chassis just to do anything to the engine, such as change out the fuel pump. I just smiled, nodded and left thinking that's about the dumbest thing I've ever heard and I knew he was confusing my truck with the newer generation trucks with the 6.4. Considering oil is designed to reduce friction I don't understand how it could be too slick. Anyone else ever heard of synthetic oil being "too slick" to lubricate properly?