Nissan doesn't give a recommended service interval for ATF replacement, even under their "severe duty" schedule." It only says to "inspect" it every 15,000 miles. Obviously, if you "inspect" it and the fluid is discolored or dirty, one should service it. Years back their recommended schedule was replacement at every 30,000 miles, which being that the factory fill ATF, Nissan Type "D" ATF, is not synthetic, it's not a bad practice to do a drain and fill at that interval, IMO. Nissan Type "D" ATF is essentially the same formula and viscosity as Dexron/Dexron II. WHile Dexron III is technically campatable and okay to use to top off the ATF level, it is not recommended for use (per Nissan) as a service replacement. Dexron III has a thicker viscosity than Dexron II and has been known to cause incidents of valves sticking in the valve body assy. For those reasons, Nissan recommends use of Nissan Type "D" ATF only. On the aftermarket, there are a few choices that are recommended for the primarily Asian-built applications that specify Dexron II. Valvoline Maxlife ATF and Castrol Multi-import ATF are two of those. Both of those are synthetic ATF's.