Many believe you can not mix synthetic with conventional engine oil. But mineral and synthetic engine oils are completely compatible. Mix the two wont hurt a thing. You can switch back and forth between synthetic and conventional oil whenever you want as long as the oils meet the manufacturers specifications. Some cars require a synthetic oil just to meet the viscosity requirements of the engine.
In my 2000 7.3L Powerstroke Ford Excursion, I mixed 6 quarts of Mobi Delvac 15W-40 Conventional Oil, in with the 9 quarts of used (2500 mile) Valvoline Premium Blue 5W-40 Synthetic Oil that was already in the truck. I've got a 5,000-6,000 mile road trip coming up in a couple of weeks, and I'll have the used engine oil analyzed after the trip to see if it made any difference.
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