Legally Nissan cannot avoid a warranty claim as long as the vehicle was serviced correctly (appropriate spec for each fluid or other consumable) as has already been mentioned.
The thing about getting Nissan to service the vehicle is consistency. As long as the Nissan dealer IS following the factory "recipe", your car will be serviced correctly and accurately every single time.
This "join the dots" methodology DOES present a problem at times. Sometimes an error code will present itself - like with a local here, Beergutz, who had a code appear that indicated something to do with the fuel pump. After analysing the actual issues, I suggested he loosen his fuel cap and the problem went away. Nissan would have replaced the fuel filter, fuel lines, fuel rail, injectors, fuel pump (there's $4K) and eventually have given up (perhaps). The manual has a strict, set order in which to replace components in order to diagnose a condition. Mind you, I'm not sure if that problem has returned.
As for different brands of oils - there's no drama in doing that, the oil only has to meet the specification. Those of us with DPFs need to ensure that the oil is JASO-FD rated (extremely low ash). They're actually more particular about the auto transmission fluid, because the torque converter is designed to drive (and stall) at particular levels for precisely the oil they specify. There might be similar oils, but I get the feeling (reading the manual) that if they spot a hint of something else in there, they'll tear the auto gearbox warranty up (and the manual does say that).
Overall, though, getting a non-Nissan mechanicto do the servicing or even repairs correctly shouldn't present a problem with warranty - just keep all the receipts and any job sheets you're given.
