I've only been researching synthetic oils since the recent purchase of my z, so take this with a grain of salt.
If you're spending money for power, put it towards tuning, intakes or exhaust....not oil. You're not paying $6-10 a quart for power, you're paying for a better quality engine lubricant that lubricates better and protects for much longer durations.
One of the few differences between Mobil 1 and other oils like RP, Redline and Amsoil, is the amount and/or quality of the additives, including the Nissan-recommended esters. Mobil 1's site says the following of their oil:
Quote:
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Many synthetic oils, including Mobil 1, contain a variety of synthetic fluids, and usually this includes esters.
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Redline, RP and Amsoil are all very open about the quality and amount of additives used in their oil.
I believe the break down is as such, based on about 10 minutes of googling (please correct me if I'm wrong):
Castrol - Group III base with (not sure what group) additives
Mobil 1 - Group III base with Group IV/V additives
Mobil 1 Extended Perf. - Group IV base
RP - Group IV/Group IV blend
Amsoil - Group IV base with Group V additives
Redline - Group V
A google search or 2 can ambigously lay out the differences between the groups, however from what little bit of evidence we have...all 3 provide sufficient lubrication for the engines in our Zs and you should not have any problems stemming from the usage of any of the above.