I always use Reline or Amsoil synthetic in long service items like the transmission or axle. Renfy, by all means change the filter at suggested intervals, empty the pan and refill with new fluid in between if you want to, but do change the filter at the correct intervals.
Mobil has been changing their formula little by little hoping that you don't notice. Mobil 1 and Castrol Syntec are not true synthetics, they are ultra refined mineral oils (hydrocraking). I don't want to get in to it, but if you want more information search the Forum on hydrocracking and additives for the whole story.

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Thanks,
Dave

1998 BMW 540i Sport
1995 E320
1992 400E
1989 BMW M3
1979 Triumph TR7 (1st car)
1986 BMW 735i (Sold)
1980 BMW 320i (Sold)