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Garage put some oil in car and after I checked the dipstick it looked black and thick. Car has since been running sluggish and the cause might be the oil(though this work followed an overheating situation so could be something else).

I did an oil change and managed to get 3.5l into the 4l capacity so that would mean .5l or 12.5% of old oil is still there. It sounded and moved a bit better after the change but is still quite sluggish.

In case that oil is the cause and the 12.5% oil can make that much difference, I need to get it out. However I do not know the viscosity of the oil they put in so doing constant oil changes may not help, since when draining the different oils will settle, not mix and hence the garage oil may settle in the area that doesn't drain out i.e. the 12.5% wont dilute and may locate in the non drain-able area. Therefore doing oil changes may not help.

Would engine oil flushes work for my situation? I've heard they can do more harm then good, any truth in this?

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  • Yes, follow the instructions... Or, just take the vehicle for a drive so yhe engine is warm, allow time for oil to drain back to sump and then drain and fill with new.
    – Solar Mike
    Oct 24, 2018 at 4:35
  • Did you change the filter when you did the oil change? If not a) the filter will hold dirty oil, and b) the filter will hold about .5l (depending on filter) of oil so you won't be able to put 4l in it.
    – Tim Nevins
    Oct 24, 2018 at 15:54
  • I wouldn’t use a flush. The deposits that have built up over time in the oil ways will be released. The oil pump will then push them to the bearings where they can block the flow of oil. Just empty the old oil and refill with new.
    – HandyHowie
    Oct 24, 2018 at 19:32

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Generally an engine flush, done properly, is a good thing. However, in an old, high mile engine where the seals aren't in great shape the old oil and gunk may be the only thing keeping it from leaking and a flush may remove that protection. So if your engine is old and hasn't seen much love it may be best to leave it alone and just change the oil.

More likely likely a flush would be a good thing, but from your description of your problems it doesn't sound like it's going to solve them. If your car overheated then there's many other things that could be wrong. No harm in trying most likely.

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  • They are definitely a "good thing" for garages. Something quick and easy to do, that you can sell to non-car-savvy customers as an "extra" to a standard service, with a huge profit margin. If you are really unscrupulous and the oil looks pretty clean before you drained it, you don't even have to do anything at all except put it on the bill - how is the customer going to know? ;)
    – alephzero
    Oct 24, 2018 at 8:28
  • For sure @alephzero, the poster seems to be talking about doing it himself, so not a concern in this case.
    – GdD
    Oct 24, 2018 at 8:30
  • @alephzero you don't have to trick "non savvy customers" just wait for them to f**k up simple jobs and then sort them...
    – Solar Mike
    Oct 24, 2018 at 10:28

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