Yes you can switch to full synthetic at 40,000 miles. The general rule is to wait until the engine has broken in. The witch craft to that is watch your mileage and when it levels out generally about 8 K miles you are free to make the change.
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Neil & Karol 2015 Seneca HJ
Dolly tow 2017 Cadillac XT5, Toads 2008 Chevy 1 ton DRW, 79 Jeep CJ5
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