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Mixing Synthetic Oil Brands

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I took my wife's car (not a Porsche) to the dealer the other day to have some maintenance done. I asked them to use synthetic oil, and they used Amzoil. If I need to add some before the next oil change, can I just add some Castrol sythetic or Mobil One of the same viscosity?

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They used Amzoil.  If I need to add some before the next oil change, can I just add some Castrol sythetic or Mobil One of the same viscosity?


It is far preferable to use the same brand and type of oil. Porsche says:


The engine oils approved by Porsche can be mixed to the extent that it is not necessary to flush the engine when the oil is changed, or another type of oil is used. This holds true for mineral oils and synthetic oils.

However, since every brand of oil has a specifically adapted unique composition, the same oil should be used for any topping up that might be necessary between change intervals, whenever possible."

Basically, if you mix two oils the oil manufacturers have to make sure that the resulting mixture does not turn into some sticky goo. However, each band has its own additive package, and for optimum performance they should not be mixed.

Incidentally, this even true within a brand. For instance, Castrol has hydrocrack (group III) based "synthetic" oils and Polyalphaolefine (group IV) based "synthetic" oils. My understanding is that the PAO-based oils use a different additive package, so for best performance the two should not be mixed.



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