The 2003-06 350Z owner's manual states: "Currently available exact equivalents to Nissan Genuine ATF Matic Fluid J are Castrol Transmax J or IDEMITSU ATF HP."
Castrol never marketed the Transmax J in the USA, so don't bother with that one. The IDEMITSU product is what Subaru sells as their Genuine Subaru ATF Fluid, and while I haven't checked the price at my local Subaru dealer, I read somewhere that Subaru sells it for about $5/qt. I own an '05 Outback, and my owner's manual lists the recommended trans fluids as: Genuine Subaru ATF, IDEMITSU ATF HP, Castrol Transmax J, and Pennzoil ATF - J.
Sidenote: Pennzoil product was never marketed, to my knowledge.
Note: The Idemitsu product is conventional fluid; whereas, the Castrol product was a synthetic blend (described on Castrol Australia website, but no longer sold there; interestingly, described as full synthetic on Castrol Germany). The Idemitsu product is only available if you have your trans serviced at a Cottman Transmission Center (MSRP price of $4/qt), or if you belly up to a Subaru parts counter. Hope this helps.