For the injectors, I'll buy a can of Techron every 60,000 miles or so if I think about it. Before I moved out of their service area, I'd just buy a tank of Chevron gas now and then.
It sounds like the dealer just wants to flush your wallet. And I think you're just flushing good oil down the drain too.
On my older Nissan, I change the oil every 7,500 miles, just like the manual recommends. Same as your Sentra per the Edmunds Car Maintenance Guide.
My Quest is at 182,000 miles btw.