0W-40 isn't what Nissan recommends. They recommend 0W-20 with 5W-20 and 5W-30 being acceptable substitutes.

I use Pennzoil 0W-20 that I buy online from oil-store.com.

http://oilstore.stores.yahoo.net/peplforhy0wc.html

Even with S&H it's about $5.60 a quart.