Vehicle: 2004 Mini Cooper (1598cc Petrol, 3-door hatchback) (United Kingdom).

My knowledge-level: Low.

Background: My oil level is below 'minimum'. I suspect a leak and have booked it into the mechanics next week - the leak is not the concern of this question. In the meantime I need to top the oil up. The previous owner (I've had the car for 4 months) had been using PART-SYNTHETIC 10W/40 (I know this because they gave me their spare oil when I bought the car). My research today has found that the correct oil for the car is FULLY-SYNTHETIC 5W/30.

Question: Should I stick with the 'wrong' oil and top up with part-synthetic 10W/40, or should I add the correct oil (thus mixing two different oils)? Note that it is below minimum so it needs to be topped up with a decent amount.

(Edit: my question is slightly different to is-it-ok-to-mix-oil-of-different-viscosity-in-a-wankel-engine as I'm also asking about mixing partly-synthetic with fully-synthetic, not just different weights)

  • Eventually, is the spare oil the ex owner gave you enough to top up? mixing grades isnt that bad for one week as long they are the same kind but since the mechanic is behind the corner you could just use what you have until then just to get rid of it, then flush and replace anyways. Commented May 13, 2016 at 14:07
  • @JPhi1618 is my question different to the one you highlight, in that I'm also asking about mixing 'fully/non-synthetic', and not just weights? Genuine question - I don't know enough about the subject to know. Then again, there may be other users with a similarly low level of knowledge who would benefit from the 'fully/part-synthetic' aspect being addressed.
    – andydavies
    Commented May 13, 2016 at 14:11
  • Hey, welcome to the site Andy. It takes 5 votes to actually close a question. I added that because I felt it was similar, but you're likely to still get an answer. I'm not sure how big of a difference the "level" of synthetic make, but the other question seems to show that sticking with the same brand (so I would assume the same type) is best just in case certain things don't mix well - but that's just me reading. I don't know...
    – JPhi1618
    Commented May 13, 2016 at 14:28
  • This does not seem to be a dupe to me. Commented May 13, 2016 at 16:48

2 Answers 2


It's fine to run Synthetic Oil with crushed Dinosaurs, which is exactly what 'part-synthetic' oils are - but because the chemistry of the two oils you have is unknown, I wouldn't do it for too long (the TBN - Total Base Number - of the mixture may change in unexpected ways).

By running the 'wrong' oil, you're better off than running it with no (or low) oil!

tldr; You're OK to add the gunk you have, and let the garage sort everything next week.

  • I agree. Especially if you have the "wrong oil" on hand. As stated, better wrong oil than no (or low) oil. @andydavies - I'd also expect the shop to put the wrong oil back into the vehicle, so expect to do an oil change when you get the vehicle back. Commented May 13, 2016 at 17:05
  • 1
    I'd like to specify an important point. While the "wrong" type of oil, e.g. Synthetic, Synthetic Blend, Conventional; is fine, and will not damage the vehicle, running the wrong weight oil will most certainly damage the motor. Motors with fine tolerances running too thick of an oil may experience premature wear/poor oiling characteristics leading to shortening engine longevity and subsequent failure. Running too thin of an oil in a motor with larger tolerances will results in poor oiling characteristics, insufficient lubrication and premature engine wear and subsequent failure. Commented May 14, 2016 at 6:06
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    Always use the recommended oil weight for your vehicle. "Type" (Synthetic, Semi-synthetic, Conventional) does not matter except in the case of Wankel (Rotary) motors which require a mineral or conventional based oil due to their oiling characteristics depending on the oil burning cleanly (read: Wankels are total loss oil systems). Many Synthetics/Semi-synthetics fail to do this and will leave behind deposits causing premature engine wear and failure. In piston powered automobiles, they are unaffected by oil type. So feel free to switch away! Otherwise, use the correct weight when possible. Commented May 14, 2016 at 6:12
  • 1
    To clarify, do not run your motor without oil - in a pinch, oil is oil, however do not think that this is an acceptable activity to perform in the long run. You should switch to the correct oil as soon as possible. Provided that your motor has not apparently had any lasting effects from the wrong oil weight (looser tolerance engines are more forgiving) being used, you may see a gas mileage increase and perhaps performance benefit from your vehicle. If you notice any new noises after switching back to the correct oil viscosity, it's possible you may have damage due to poor oiling. Commented May 14, 2016 at 6:20

Rather than mixing I would purge the wrong oil out of the system and fill up with the correct oil. Since you say it's below minimum you won't be draining out too much of it. If your skill level doesn't make you feel confident with this operation then you could fill up with the oil you have at hand and try not abusing (or using) the car too much before your trip to the garage.

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