Castrol oil is made from a better quality base stock than Mobil. There is only one type of Mobil oil which is suitable for the GTI and thats the ESP one, non of the other Mobil oils are approved by VW.
Mobil oils comply with the american API standards, whereas Castrol conforms to a european standard. The euro standard is much more stringent and up to date. For example a euro fully synth has to be just that. But the american standard for fully synth still allows for some mineral oil in there.
In short, if you have the choice go for the Castrol. Or Fuchs.
Don't get me wrong, the mobil ESP stuff will do you
no harm whatsoever but the castrol stuff will be better
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