I have a 2005 petrol C4. The car is supposed to have 5W-40 in it at the moment. I have a tiny oil leak which would cost as much as the car to fix, so the cheaper solution is to top-up the oil every now and them.

Yesterday in the store I was browsing the oils and they had "Text your reg to this number to check which oil" so I did and the response suggested I buy the 10W-40. Not thinking about anything I did it and just now it hit me that I'd be mixing different types of oils.

From browsing left and right I come to understand that as long as the second number doesn't change I'd be fine, but not much is written about synthetic and semi-synthetic oil mixtures.

So can I add some semi-synthetic 10W40 to my synthetic 5W40 ?

2 Answers 2


It's ok in a pinch. The 50 IS much thicker. Try not to fix things AND regular. Symethic is made of smaller molecules. It can get to areas regular oil can't.

  • So If i top-up some 10W40 and later down the road I topup both 5W40 and 10W40 I'd be fine ? Sep 23, 2016 at 7:33
  • Doesn't synthetic and conventional oil of same viscosity still have different temperature ranges?
    – method
    Sep 23, 2016 at 23:45

Check the manual many cars have a range of acceptable oils dependent in part on ambient temperature, but small amounts would be fine in any case.

The big considerations are low temperatures, when the 5W40 would flow better than the 10W40 and high temperatures, like a turbo – especially after engine shutdown – where the synthetic would be less likely to "coke up." If you don't have a turbo, many fewer worries.

If the manual specifies synthetic oil, take the seriously. Synthetic is more expensive (but that cost may be made up with longer oil change intervals) and the maker would probably rather not impose that cost if they could avoid it. Before you go against the manufacturer's advice try to understand their reasoning and decide if the risk is acceptable to you.

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