Is it generally OK to mix synthetic and non-synthetic oil in the same engine?
My 1999 406 (2 litre petrol) needs topping up but I have no idea what is sloshing around in there at the moment.
I assume that synthetic would be the better purchase, but am just unsure as to what will happen if its standard mineral oil in there at the moment.
I have read the excellent guide to oil here www.honestjohn.co.uk/forum/post/index.htm?t=42671 but it doesnt clarify what happens if the two types get mixed.
Any advice appreciated
Thanks,
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You can mix these oils, but as a semi-synth can now be bought for the same price as a good quality mineral oil, a top up with semi-synth would be a better option. Change the oil and filter in good time.
659.
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They do mix, there's no know problem, but there are no tests and no official data on the subject anyway.
It's not best practice, and it's one of those things where you're on your own.
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When mixing oils always assume that the result will be the equivalent of the lower grade used.
Not strictly true but its a damn good guide.
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The Castrol tech support people told me that their oils are fully mixable ("miscible"). They started off saying that all oils were fully mixable, but then restricted the advice to their own range, only out of corporate ultra-cautiousness, I suspect.
In case anyone hasn't yet been to the site giving you more than you will ever want to know about oil, here it is:
www.carbibles.com/engineoil_bible.html
It offers very clear explanations.
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Great, thanks for all your advice.
Paul
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