I can't understand the conflicting viewpoints on engine oil and lubricants. It seems to me that the investment that was made in getting a deisel engine, people would want to spend a little extra and get quality lubricants. Unfortunately you can teach an old dog new tricks is true in this case. How is changing your engine oil every 3-4000 mile good? It's just what the oil companies want and it costs you bucks in the end. Regular petro based oil is garbage and the current crop of synthetics are no better. I tried Mobile 1 and it doesn't last. The chemistry is their. the oil companies like to keep it for the future so they can say gee whiz look at what we have. I've used AMSOIL since day 1. I went over 20000 miles before an oil change, but I had it analyzed. Result was no degradation in quality, continue to use. All I did was change the filter a suggested. I have a dual by-pass filtration system. I now have 61thou miles and plan on using this oil for over 50thou. I also plan on having it analyzed. I use Synth. in the transmission and rear end, and power steering, and it works fine. I believe it's the chemistry of the oil that makes a superior product and not what name they give it. A friend of mine put AMSOIL in his Toyota truck and now he swears by it.