I think that my vehicle would probably benefit from having it's lubrication system being flushed out at it's next oil change. Checking prices of flushing oil at local suppliers I noticed that they are now also selling a variety of flushing additives (such as STP Engine Flush).

With flushing oil I would normally drain old oil, half fill with flushing oil, run the engine at a fast idle for a 15-20 minutes to circulate it, drain it again, change oil filter and finally fill up with normal oil. With the additive you don't need to drain the oil twice however, you just add it to the old oil in the engine, run it for 15-20 minutes then drain it out with the old oil and change the filter/oil.

Overall the additive option will be more expensive (£4 per flush rather than £2 per flush for my engine) but will take less time (one complete engine drain/fill rather than two). It would also be more environmentally friendly, as there would be less waste oil to dispose of.


So, aside from the cost/time/waste concerns, is the additive solution as effective or even more effective than using traditional flushing oil? Which results in a cleaner engine? I.e. less gunk from the old oil in the new oil.

  • It seems strange that the oil is thick and gloopy if you are frequently topping it off because it is being burnt up.
    – jzd
    Jul 26, 2011 at 11:22
  • Many people recommend putting seafoam in the crankcase. Apr 18, 2016 at 9:52
  • As I said to FossilizedCarlos, recommending a brand only available in certain locations is not very useful on an international site @RobertS.Barnes, which is why I couched my question in more general terms.
    – Mark Booth
    Apr 19, 2016 at 8:25
  • It's fairly easy to make homebrew seafoam, I've done it myself: mechanics.stackexchange.com/a/16491/7132 Apr 19, 2016 at 8:43
  • I'm not sure how this helps me improve my question, but it sounds like you have the seed of another answer to this question.
    – Mark Booth
    Apr 19, 2016 at 9:10

3 Answers 3


The main difference between the two is that flushing oil is extremely thin oil that theoretically should be able to get into partially blocked, thin passages without any issues and remove the accumulated gunk there as well. Oh, and out any slightly leaking gasket, too. It's usually also got a suitable cleaning additive if you've bought a good oil.

The additive doesn't really change the viscosity of the existing oil in the engine and is supposedly "just" a concentrated detergent package that you'll find in your regular motor oil (and you can hope that there is no chemical reaction between the two - there normally isn't, but there might be a chance for that). If your oil is already holding a lot of contaminant in suspension and the oil filter is reaching its filtering capacity, then you might to more harm than good.

I've used both, and I still (very) occasionally use flushing oil. However, for a decent running engine that hasn't suffered from a lot of contamination, I tend to prefer the third option - change the oil a couple of times in fairly short intervals, use a high-detergent oil (I like using diesel-specific oil that's also got the correct classifications for petrol engine, as that has a lot more detergents in it than the petrol engine variety) and change the filter every time. Oh, and use a high quality filter so it actually traps the contaminants that are cleaned out by the oil.

  • Thanks Timo, given that the flushing oil I can find around here isn't much cheaper than regular oil, I might well be better off taking your third way. *8')
    – Mark Booth
    Jul 26, 2011 at 18:20

Unless your Engine is gunked up, and black sludge is prevalent, do not perform a flush. Just keep a regular maintenance of regular oil changes, and you will be fine. Flushing oil can do more damage to your Engine if your engine does not need it. I had a 150K Diesel VW Golf which I like a twit decided to do flush on, this caused it to smoke profusely. It had been fine previously to this. only after 10K did it stop smoking I used a BG flush item, with allegedly ring clean Technology. Never again. DO not tinker with a running engine just perform a regular oil change with a filter. No additives. Today's cars run with Long Life fully synthetic oil. Black sludge in old mineral oil in the 80s and early 90s with a poor breather assembly did suffer from this. If you have a dirty petrol engine, place a oil made for diesel and run it for about 50 miles and drain. The Diesel oil has surfactants and cleaning additives which clean and shift gunk. But do be careful.

  • Thanks Jeeves, some nice information here, but it doesn't really answer my question.
    – Mark Booth
    Jul 24, 2016 at 15:40

Have you tried sea foam? I use Castrol Syntec, and run sea foam before my oil changes. I run it about 40 miles, which is the trip to where I change it, and it keeps my engine clean. I'm not sure how good it would work if the system is already dirty though.

  • I don't think that particular brand is available in the UK, which is why my question was asking about flushing oil vs additives rather than about specific brands of each. Also, any product that is simultaneously promoted as a fuel additive, engine oil additive and brake fluid additive sounds a little like snake oil to me. *8')
    – Mark Booth
    Jul 27, 2011 at 8:39
  • @Mark Oh... It didn't even register that you were talking in £s. I apologize :) I do use it though, and it works well. I was sketchy at first, but a guy at work (We were both mechanics) had been using it for a while. He recommended it, and I have been happy since. Jul 27, 2011 at 12:01

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