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Thread: Mixing synthetic oil and "regular" oil

  1. #1
    WalterD Guest

    Question Mixing synthetic oil and "regular" oil

    I know someone who had a friend (not mentioning name) who might have "accidentally" used about 2 quarts of "regular" SAE 30W Pennzoil in an engine that had synthetic oil in it. There were some minor oil leaks, over a period of time, that is why it was low. Is this a problem? Can changing oil and filter make it all better again? Does the engine need to be flushed? Does my friends, friend need to go back to synthetic or regular oil. Coincidentally the motor is a '79 304. It had about 50 miles on the engine with the mixture of the 2 types of oil. The engine does not smoke, it just has a few more leaks (not major) or so I've been told. Any help for my friends friend will be helpful.
    Thanks Walter

  2. #2

    Re: Mixing synthetic oil and "regular" oil

    Not a problem. They make conventional and synthetic blends.

  3. #3
    Join Date
    Sep 2005
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    Re: Mixing synthetic oil and "regular" oil

    You now have synthetic blend in there.

    Kyle

  4. #4

    Re: Mixing synthetic oil and "regular" oil

    Walt -

    It probably isn't anything to lose sleep over.

    Mobil states that it can be done and is okay

    http://www.mobiloil.com/USA-English/...entionals.aspx

    Of course if a pure synthetic, that appears not to be compatible:

    http://www.carbibles.com/engineoil_bible.html

    Mixing Mineral and Synthetic oils - the new hotness
    That's the thing with progress - stuff becomes out-of-date. Fortunately for you, dear reader, the web is a great place to keep things up-to-date, so here's the current thinking on the subject of mixing mineral and synthetic oils. This information is based on the answer to a technical question posed on the Shell Oil website.
    There is no scientific data to support the idea that mixing mineral and synthetic oils will damage your engine. When switching from a mineral oil to a synthetic, or vice versa, you will potentially leave a small amount of residual oil in the engine. That's perfectly okay because synthetic oil and mineral-based motor oil are, for the most part, compatible with each other. (The exception is pure synetics. Polyglycols don't mix with normal mineral oils.)There is also no problem with switching back and forth between synthetic and mineral based oils. In fact, people who are "in the know" and who operate engines in areas where temperature fluctuations can be especially extreme, switch from mineral oil to synthetic oil for the colder months. They then switch back to mineral oil during the warmer months.
    There was a time, years ago, when switching between synthetic oils and mineral oils was not recommended if you had used one product or the other for a long period of time. People experienced problems with seals leaking and high oil consumption but changes in additive chemistry and seal material have taken care of those issues. And that's an important caveat. New seal technology is great, but if you're still driving around in a car from the 80's with its original seals, then this argument becomes a bit of a moot point - your seals are still going to be subject to the old leakage problems no matter what newfangled additives the oil companies are putting in their products.
    Some day, One Day, my Jeep will no longer need new modifications

  5. #5
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    Re: Mixing synthetic oil and "regular" oil

    Quote Originally Posted by WalterD View Post
    I know someone who had a friend (not mentioning name) who might have "accidentally" used about 2 quarts of "regular" SAE 30W Pennzoil in an engine that had synthetic oil in it. There were some minor oil leaks, over a period of time, that is why it was low. Is this a problem? Can changing oil and filter make it all better again? Does the engine need to be flushed? Does my friends, friend need to go back to synthetic or regular oil. Coincidentally the motor is a '79 304. It had about 50 miles on the engine with the mixture of the 2 types of oil. The engine does not smoke, it just has a few more leaks (not major) or so I've been told. Any help for my friends friend will be helpful.
    Thanks Walter
    No way is this a problem. The only thing that is bad is when a motor is broken in and has used a certain oil (either synthetic or non-synthetic) for a majority of its life. Sometimes when switching from either non-synthetic to synthetic or vise versa it causes leaks with engine gaskets or oil plugs. Dont ask me why, I believe it has something to do with the gaskets getting used to a certain type and when you change they say "***" and start leaking just to piss you off... My unique way of explaining it...

    Here's a little tid-bit of information: In aviation motors, they come from the factory with mineral oil. After the consumption of oil stops or approx. 50 hours of run time (motor is broken in) they recommend you switch to an ashless dispersent for the rest of the engines lifetime.

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