I think the G35 calls for an energy conserving oil. It's hard to tell whether that's just for fuel economy or whether there is a bigger issue like catalytic converter compatibility.
I found a few excerpts of the owner's manual with a quick Google search:
Quote:
If you cannot find engine oil with the API certification mark, use an API grade SG/SH, Energy Conserving I & II or API grade SJ or SL, Energy conserving oil. An oil with a single designation SG or SH, or in combination with other categories (for example, SG/CC or SG/CD) may also be used if one with the API certification mark cannot be found. An ILSAC grade GF-I, GF-II, GF-III oil can also be used.
In other words, they really prefer if you use an "energy conserving" oil, but it's not absolutely essential
Now, here's what Castrol says about GC:
Quote:
Exceeds all car and light truck manufacturer’s warranty requirements for the protection of gasoline, diesel and turbocharged engines where API SL, SJ, SH, CF or CD is recommended. Exceeds European ACEA: A3, B3, B4; VW 502 00, 505 00, 503 01; MB 229.1, 229.3, 229.5; BMW LL-01; GMLLA-025, GM-LL-B-025 and the engine protection requirements of ILSAC GF-3 for API Certified Gasoline Engine Oils.
From my reading of this, GC will protect your engine well enough. I strongly doubt it'll be harmful. The worst you can say about GC is that you'll lose some fuel economy, and you can probably find something that'll provide enough protection while also delivering better MPG.
I'm not sure about the Maxima, but I definitely would bank on it being similar.