I've seen that happen lots when off-roading in several different vehicles. I wouldn't worry about it at all, its just from the steep hills. The worst was in my Cummins Ram which had a crackcase vent (draft tube) at the front of the head. It would literally dump oil into the fan on a steep downhill (and thus splatter all over the engine compartment). On one particularly nasty offroading trip, I had to put 1.5 gallons back in it afterwards. Luckily, Nissans (and Jeeps) don't have a similarly stupid design.