One of the Biggest Problem's with the Nissan Midsize Truck, is Young people that buy them and Abuse them in Mud, OffRoading, and Water, because they are the cheapest Midsize Truck. Sound's like you got one that was abused.
Someone will be along shortly with answer's to your specfic question's.
The Owner's Manual,, tell's you which Oil's and Fluid's to use for your truck.
If your truck is a V6 with AT, read up on Coolant Contamination with AT Fluid.
2013 SV V6 AT 2WD KC Frontier/Current; '93 Hardbody KC,IL4,AT,2WD, O.O./20yrs./Sold