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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?

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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 at 18:24
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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
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    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
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    @eddyce - That is a fallacy. What do you think synthetic blend oils are? – Pᴀᴜʟsᴛᴇʀ2 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) – Pᴀᴜʟsᴛᴇʀ2 Aug 6 '17 at 14:53

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