Can I mix two different brands of car oil?
I have a 2012 Ford Fiesta, and used GTX Ultraclean 10W-40, however I couldn't find this one again so I might Mobil 10W-40. Is it okay to mix the two brands? 
No you can't.
Yes, as long as it's the same viscosity (10W-40), then it's fine.
Yes, all motor oils are blends oil multiple base types, and can be mixed.
But why are you using 10w-40 in a 2012 Ford? The owners manual specifies 0w-20.
No problem, I do it all the time…
Yes you can mix different brands. What you should not mix is real petrol oil and synthetic oil.
Yes
That's fine.
Yes. Your engine won't know the difference, or care.
Youa re usingthe WRONG viscosity for that engien! I would getPENNZOIL PLATINUM or PENNZOIL EURO l That MEETS the needs of your particular engine! Yoru engien is EUROPEAN< so you need FULLY EUROPEAN FULL SYNTHETIC OIL! Use LIQUI MOLY if you can find it! CASTROL is NOT my favorite! Int eSting it is NOT as good as say TOTAL ENERGY OIL from FRANCE! ANd VALVOLINE is a "NO GO" formt he beginning!
Absolutely, IF you're using the correct engine oil viscosity which is 5W-20.
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