2006 F250 6.0 L Powerstroke

[Donajo]

Recommended oil for F250 6.0 L

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I have a 2006 F250 6.0 L Powerstroke with 130,000 miles and it has been a great truck I have always used Rotella 15/40 at every 5000 miles. I would appreciate if any more knowledgable members could help with my oil change decision. i am considering on the next oil change going to Valvoline Premium Blue and would appreciate your opinion. Also, if you go right to a snythetic from always using a non-synthetic is there any issues with seals leaking. due to changing to a snythetic. I had a mechanic here tell me you need to do half synthetic and half regular at first.
Thank you