I'm real new to diesels so anything I opine about is speculation. BUT, I have been using Mobil One since it was $1 a quart in gas engines. I have also called Mobil with questions. They say you can mix it, stop using it, start again, put it in high mile engines for the first time with no probs. That rumor that putting Mobil One in a high mile engine and creating leaks is BS. I've done it. The problem you may have is clogging the oil screen with loosened crud, since M1 evidently has more or maybe a different detergent in it.
Those stories are from the 60's when synthetics first came out and gasket/seal technology of the time didn't always like it. We are in a whole different place 50 years later. You can mix synthetics with dino oil as is done with the synthetic blends, you can switch back and forth between dino and synthetics as often as you choose.
The thing you want to consider in this though is the fact that synthetic 5w40 has proven to be beneficial to injector life in the 6.0 Powerstroke. Dino oil leads to varnish buildup on the injector spool valve and causes what we call stiction, where the injector sticks and no longer works correctly. This is the leading cause of injector replacement in the 6.0.
Just something you want to consider when choosing a motor oil. We have had good results fro Rotella T6 (available at Walmart for $21.xx/gal) Valvolene Extreme Blue, Mobile1, and Amsoil. 5w40 is the preferred weight and can be run year round.
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