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Let's say that I just replaced my engine oil but decided I want to lift the valve covers to do some gasket replacement. I know it may sound cheap but I'd rather not throw away $25 worth of almost new oil. Would it be safe to catch the old oil in a receptacle and set it aside and put it back into the engine once I am done with my repair?

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As @dodgethesteamroller stated, there is nothing wrong with reusing the engine oil. There are two caveats and one concern I'd like to mention with this.

First, use a clean container to store the oil in while you are doing your engine work. Your normal engine drain pan will probably not give you the cleanliness you are looking for. You DO NOT want to have the dirt and crud which collects in the pan to be put into your engine.

Second, it's probably a good idea to change out the oil filter when you are doing this. It doesn't hurt anything and should only cost you a few dollars to replace.

Concern: Before you decide you are going to reuse your nearly new engine oil, you need to realize the reasons for doing the work on the engine in the first place. If you are replacing worn bearings, you don't want to reuse your engine oil no matter how new it is, because it will more than likely have metal fragments from the old bearings in it ... no sense in putting metal particles back into your engine and having all those wear items thrown into the mix. It just does not make sense.

I don't blame you for wanting to save a buck and/or the environment, but don't ruin your new engine work in the process by not utilizing some common sense.

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    Just as an addition to this, reusing the oil really only makes sense if it really is nearly new.. a couple of months later, the risk isn't worth the cost. – Leliel Jun 3 '15 at 3:47
  • @Leliel - I couldn't agree with you more. I personally would not reuse engine oil, even if I had just put it in the engine. – Pᴀᴜʟsᴛᴇʀ2 Jun 3 '15 at 11:18
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    @Paulster2: Really? Not even if it was brand new? I think that's extreme, not to mention wasteful. Conventional oil is a finite resource, and synthetic oil is pretty expensive ($5-$8/qt in the USA)--not that conventional is exactly cheap these days! Say you had just changed the oil in a car with an average-sized sump--5 quarts--and then, like the OP, decided you needed to do some work on the engine that would require draining the oil. If you'd literallly throw $20-$40 down the drain at this point to avoid reusing the oil, you have more cash to burn than most people. – dodgethesteamroller Jun 3 '15 at 11:43
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    @dodgethesteamroller - What is the bigger waste of time and resources, replacing semi-used oil which may or may not be useful, or replacing an engine because it wasn't? I look at the potential for loss vs. money expenditure. Not that I'm trying to be wasteful or anything, if I put new oil in, I know it's new oil which will not have any issues. If you have to drain the oil to do work on the engine in the first place, you are opening the bottom end of the engine. This means there is something wrong there, which is where the oil resides. I just am not willing to take that chance. – Pᴀᴜʟsᴛᴇʀ2 Jun 3 '15 at 12:26
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    @dodge - Paul explicitly says in his answer that it is almost certainly fine. It is only in these comments that he goes into his actual opinion. I'm not sure what your comments or flag are trying to accomplish, and I'd suggest rereading his post and comments to see why you are mistaken. – Rory Alsop Jun 6 '15 at 9:28
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Nothing wrong with doing this at all. Just make sure that you keep the oil clean and put back in as much as you take out!

Usually you don't think about keeping oil clean as it drains since you're going to dispose of it anyway, so take precautions: use only new or thoroughly cleaned funnels and drain pans, wear clean rubber gloves as you unscrew the drain plug, and clean the surfaces of the engine below and around the plug as well as you can beforehand, if your car has a tendency (like most do) to dribble some oil partway down the block or along the pan when you drain it.

It would be a good idea to keep the "almost-new" oil in an airtight container with as little air space above the level of the oil itself if it's going to be out of the engine for more than a day or so, to minimize absorption of moisture from the air. (Don't worry too much, as a little water in the crankcase does no harm--it's generated all the time as a byproduct of combustion and normally just boils off when the engine gets hot.)

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    As a note, engine oil is not hygroscopic like brake fluid is and won't absorb water. In fact it will separate out if left over time (oil on top, water on the bottom). I'm not saying an airtight container is a bad thing or shouldn't be used, just that you should have no fear of the oil absorbing water. – Pᴀᴜʟsᴛᴇʀ2 Jun 3 '15 at 0:13
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    @Paulster2 Thanks for the correction. I guess I was thinking of the condensation that might cause water to collect in the airspace in an imperfectly sealed container--which, as you point out, doesn't actually mix with the oil, though it still could find its way into the engine easily by accident. – dodgethesteamroller Jun 3 '15 at 5:09
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While new oil is always preferable it is entirely possible to drain the old engine oil out and run it through some fine filters (even like one or two of those paper paint strainers or an old cotton t-shirt and then use it to fill to back to the proper level. Old oil is better than no or to little oil and better than old oil is new. Just my opinion anyway. Hope it helps

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