5W-30 fully synthetic engine oil exists in a Vauxhall Insignia engine but it needs topping up and I believe it should have oil in 5W-40 viscometrics.

Is it safe to top up fully synthetic 5W-30 with fully synthetic 5W-40 without causing damage to the engine?


Yes, both are fully synthetic and you should check that they meet similar specs (api etc).

When it next gets changed use the correct grade.

  • What does API mean?
    – Geremia
    Sep 27 '19 at 18:24

Yes, and if you were in an urgent situation, like a trip, you could top off with a non-synthetic if needed. The biggest issue with non-synthetic oils is normally around turbocharger temperatures oxidizing or otherwise polymerizing the oil.

If you were in an area of extreme heat, such as a desert area, it would be nicer to have the 5W-40, but for most areas, there will be little functional difference with the 5W-30. The -40 and -30 are numbers which describe the high temperature viscosity of the oil, with 40 remaining "thicker" at higher temperatures than the 30.

  • I was once told that you are never supposed to mix synthetic with non synthetic oils
    – Edoardo
    Aug 6 '17 at 13:27
  • 2
    You may have been. There are different properties of both. However, faced with running low on oil, it would be better to add non-synthetic oil, than to be running low on synthetic oil. Using a mix for a short period of time, say a couple of thousand miles to the next oil change, will not measurably harm the engine if the vast majority of automobiles.
    – mongo
    Aug 6 '17 at 13:31
  • 2
    @eddyce - That is a fallacy. What do you think synthetic blend oils are? Aug 6 '17 at 13:32
  • @Pᴀᴜʟsᴛᴇʀ2 a mix of synthetic oils? otherwise what's the adjective for? :)
    – Edoardo
    Aug 6 '17 at 13:39
  • @eddyce - Exactly :o) Aug 6 '17 at 14:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.