I just dropped 75.00 for 14 gallons and everyone at the gas station was shocked and pissed. "It took you 10 mins to put $75 in the little ass car!" Oh it was great and will last me about a week.
IMO If you worried about fuel economy don't do they swap yet.
350Z's use VQ3.5 300ZX use VG30's I think.
I have a VQ35 powered maxima as a daily and the gas mileage is good.