If it's the V8 you have i.e. the N62 engine then I always use Castrol Edge, BMW used to use this in their engines but have now swapped to Shell Ultra (at least here in the UK) this was down to nothing more than contract politics etc, bottom line is hopefully your doing 4-6000 mile oil changes rather than the BMW recommended distances in which case you can use what ever oil you like so long as it's BMW LL-04 for UK/Europe and LL-01 for US, everyone will have their favorites, you can ask 50 people this question and get 50 different answers but I'd buy brand name oils rather than supermarket own type oils if you get my meaning.
As for the gasket, it depends on what engine you have but again, if it's the N62 it can be quite involved BMW books this as an 8 - 10 hour job, research is key here to make sure you get all the right bits as you'll probably be replacing more than just this gasket during the process of this job.
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