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I just replaced the oil in my 2004 Jeep Wrangler today - I thought I'd give synthetic oil a try because there was a "Mobile 1 oil and Bosch filter" deal at my local auto shop.

Until now I have always used off-brand regular oil and FRAM filters, which have done their job just fine.

After putting the synthetic oil in my jeep the engine seems runs noticeably quieter than it did before. Is this because of some property of synthetic oil that I don't know about? And even though I do have very good hearing I am open to the idea that this is just my imagination.

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It's not necessarily the oil. You've also changed the oil filter you're using. It's possible the Bosch (either by itself, or in combination with the synthetic oil) allows more flow and better lubrication. I'm unfamiliar with current Fram and Bosch products though, so I don't know if there's any real world difference between them.

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    NOT RELATED: You seem like someone who might like this - area51.stackexchange.com/proposals/42572/aviation
    – Sponge Bob
    Commented Oct 9, 2012 at 4:54
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    and/or maybe the oil level wasn't correct before the change.
    – mac
    Commented Oct 10, 2012 at 16:34
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    +1. Noisy lifters can often free up when induced to do so with different viscosity/quality oil. That's pretty much what Seafoam and the like do.
    – Nick
    Commented Feb 10, 2013 at 22:02
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If the change in noise level was real and not imagined, it was likely due to the new oil being cleaner and of the right viscosity.

Besides being depleted of detergents, old oil also has a different viscosity than new oil. Depending on how the oil is engineered, and whether or not it's contaminated with fuel, the oil may get thinner or thicker with age. The oil viscosity definitely affects mechanical noise levels.

Some engines are notorious for being noisy on dirty/old oil. When my old Miata needed an oil change, the lifters would tap very loudly. Replacing the oil would always make the noise go away.

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Usually synthetics tend to make engines run louder because they have better flow characteristics.

I would bet that the oil filter was the cause of the issue, not all filters are made equal and fram makes some of the worst ones. I can't remember the article I read which took all the major filter brands and products and gave a cross section and breakdown but I'm pretty sure Bosch makes good quality filters.

To see the breakdown of filter see Oil Filters Revealed

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