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I know that sounds like a weird question but I was talking to my brother in law about oil changes and I mentioned that I almost always use Mobil 1 full synthetic. He replied, "make sure you keep using synthetic, once you go synthetic, you can't put regular oil in that engine or it'll mess it up." That sounded strange to me but I didn't know how to counter so I just left it at that. Is there merit to that claim? Does anyone have any idea how he may have come up with that idea? If so, then why?

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This is a myth. A lot of old-school mechanics and non-chemistry educated people repeat this false myth.

When changing from synthetic to regular oil, there is not anything special that you need to do because synthetic oil will mix directly with regular oil of the same weight (no engine flush is needed).

There is some differences between synthetic and regular oil. Synthetic oil has detergents and chemicals to protect the longevity of your engine.

ericthecarguy.com has a great YouTube video that talks extensively about the differences of synthetic vs regular oil and answers lots of common questions:

[2013-02-04 Regular Oil vs Synthetic Oil -EricTheCarGuy]( http://www.youtube.com/watch?v=3zEBI1QndPo)

A Valvoline scientist publicly stated switching from synthetic oil to regular oil not do anything to your engine. Additionally Valvoline has a public website that states this myth is false https://www.valvoline.com/about-us/faq/synthetic-oil-myths :

*MYTH: Once you’ve used synthetic motor oil you can’t switch to conventional motor oil. * Not true. Synthetic and conventional oils are compatible, so it is not harmful if you decide to switch."

Even Mobil1 reps agree with it. Trust me, this has been such a topic of issue; They just decided why not put the myth to the test. So, you should be fine. Your engine may get dirtier quicker, and the oil should be changed more often, but other than that you'll be fine.

update- After researching what Paulster2 commented on my reply, he is right. You shouldn't need to do a flush. He also commented a link to a guy is well versed in the subject. So I was wrong about that.

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    While you are right about it being a myth, you are wrong about synthetic being almost the same as non-synthetic. It is completely different in how it is derived. Bob is the Oil Guy has a pretty good writeup on how it's different. Also, you really don't need to do a flush. Synthetic will mix directly with non of the same weight. Update your answer and I will change the negative to a positive. – Pᴀᴜʟsᴛᴇʀ2 Jul 9 '16 at 17:11
  • Very interesting! I've been told that myth by two different service advisors at my local Dodge service department in Bellevue, WA. It didn't sound right to me either so I thought I check it out. Now I suspect that they say that because they want us all in there for service every 3,000 instead of 5 or 10. Hard to believe that they'd stoop that low. – Tom Morten Oct 17 at 19:42

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