Jon,

Unless the previous owner used a heavy duty diesel oil such as the Delo 15W40 or Mobil 1300 15W40 (both excellent compared to Rotella) then the engine will start cleaning itself, so be sure to change the filters early and often the first few times! 1000 miles or so. I don't see a reason NOT to mix them. He's saying that(probably) because it will conceivably loosen less deposists at a time. Plenty of people change to 100% syn all at once without trouble though. Whatever you do - Delo or syn - change the filter BEFORE the first oil change, unless the first oil change is at 1000 miles... You may want to run Delo 15W40 for 1000 miles once or twice and then go all syn...

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Brian Toscano