Title pretty much explains,

I change my oil every 4-6k miles depending on how generous I’m feeling, however will the oil dirty oil that’s already in the sump mix with the clean oil when topping up?

Normally I just top up but in this case I have had to drain about half a litre out on two occasions

First when I changed the oil as I overfilled

Second when topping up (overfilled again after 3k miles)

so it's had quite a bit of new oil mixed with the original stuff, just wondering will the new stuff mix fine with the old, if not I’m going do an early change and try not to be such a dope when topping up.

  • Just to be clear, when you change the oil, do you remove the sump plug and let as much drain out as possible, before you put the new in?
    – HandyHowie
    Feb 11, 2016 at 8:45
  • yeah, every 4-6k miles it's a complete drain and new filter.
    – ceefax12
    Feb 11, 2016 at 17:20

3 Answers 3


Mixing new with the old will cause you absolutely no issues. As long as you are using the same weight oil, it will mix up and you'd never know it. Continue to change your oil at the specified interval and you should be golden.

  • 1
    The manufacturers interval states every 12k miles =/ that seams way too long to me so I went with a compromise on every 6k.
    – ceefax12
    Feb 14, 2016 at 16:07
  • @ceefax12 - The 12k miles is their recommendation ... there is nothing saying you cannot do it sooner than that. This action does not void the warranty or anything, so good on yah. Feb 14, 2016 at 17:44
  • I was too worried about this But, the maintenance light/engine oil is still showing. What to do? Nov 14, 2016 at 17:21
  • @MasudRahman - It seems you have a separate issue. Please feel free to ask this as a question so it will get some deserved attention! Click here to start a new question. Nov 14, 2016 at 17:39
  • @ceefax12 What YMM? I would be very surprised to learn of a 12k Mile Severe-duty Interval. Is it a Diesel?
    – NitrusInc
    Apr 9, 2018 at 12:06

Usually when changing the oil you won't be able to fully drain the old oil anyways and the old and new oils will mix, like it or not. Usually this is not a problem even if the weights are different, though it's safer to stick with the same brand, as some argue the components in the oil being incompatible may cause issues.

What you should be wary of is mixing synthetic oil with mineral oil, it i said that this causes problems more often.

  • I have heard it said about not mixing synthetic and mineral oil and never looked into whether it is true. Isn't that what semi-synthetic oil is? Do you know what the difference is?
    – HandyHowie
    Feb 11, 2016 at 8:33
  • 1
    @HandyHowie As far as I know it's exactly that - mineral oil with some part synthetic oil. Might be an urban legend, but compatibility issues between natural and synthetic oils are said to be more pronounced. When semi-synthetic blends are made the manufacturer probably knows what to mix and what not to mix. Probably won't kill the engine and it's better to add some mineral oil than to drive low on synthetic, but this could reduce destabilize the oil and reduce it's performance. Feb 11, 2016 at 8:42
  • 1
    I personally think it's not that much of a big deal, mixing brands is alright and mixing synthetic with mineral is alright only if there is no other option. Feb 11, 2016 at 8:43
  • @HandyHowie is spot on. There is no issue with mixing Dino oil and synthetic, as that's what the "blend" oils are all about. There are no compatibility issues with doing so. This will not cause more sludge or have a chemical reaction of some sort. Feb 12, 2016 at 3:35
  • @ᴘᴀᴜʟsᴛᴇʀ2 any references? It's a very controversial subject, so I would rather err on the safe side. Feb 12, 2016 at 8:38

Mixing old with new won't hurt anything. If your engine is consuming too much oil, for instance, you would need to add new oil more frequently because you need the right oil level all the time. I have got cars that needed as much as 1L of new oil every 1 or 2 weeks because the oil piston rings where kaput. Nothing happened except $ going down there as frequently as not eager to :)

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .