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Thread: mixing regular and synthetic gear lube-- problems?

  1. #1
    Join Date
    Nov 2003

    Default mixing regular and synthetic gear lube-- problems?

    Anybody ever mixed synthetic and regular gear lubes and had problems? Just wondering...

  2. #2
    Join Date
    Dec 2007
    southernmost NJ


    New outboards come with a petroleum based gear oil in the gear case. After the first 20 hours of use, I drain them and put a synthetic gear oil in. I don't flush the gear case and there is always some residual petroleum left that mixes with the synthetic. I've been doing it this way for about 5 years and never had a problem. I don't think however, that mixing larger amounts of the two would be a good thing, even though most synthetic gear oils are a blend and not 100% synthetic.

  3. #3
    IOWOLF Guest


    As always revert to manufactures recommendations.

  4. #4
    Join Date
    Dec 2002
    Friesland, Netherlands



    We run several gas turbines offshore, and have changed over to synthetic lube oils. We checked with the lube oil manufacturers; they told us that regular and synthetic can be mixed in any ratio without problems - just that the mix will be less effective than pure synthetic (but better than regular).

    No problems with the oil turning to a gel or whatever - I think that was only when adding mineral oil to a vegetable oil like castrol R...


  5. #5
    Join Date
    Apr 2001


    Shouldn't be a problem these days. Some early synthetics weren't compatible with petroleum, but the ones you get now generally are.
    Try to make a living, not a killing. -- Utah Phillips
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  6. #6
    Join Date
    Apr 2007
    South Texas

    Default What does "SYNTHETIC" really mean?

    My understanding is that the term "SYNTHETIC" doesn't mean that the oil is created from something other than petroleum, but rather that it is perhaps hyper-refined and then the components are put back together for a specific application. You are still mixing oil & oil. The only thing I have ever heard of where there could be a problem would be from mixing parafin based (Pennsylvania Grade) oil with acid base oil (most of the non-Pennsylvania Grade). Other than that oil is oil. Unless you are really pushing the limits of the lubricating or heat transfer properties you will probably never see any difference.

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