Unfortuneately, 90% of the replies to this post are incorrect information . I've done pretty extensive research on engine oils and have found the following:

1. Castrol GTX is the best non synthetic oil.
2. FOR NON SYNTHETICS ONLY The smaller the difference between the two number on the oil, the better the oil will withstand wear and tear. I.E. 10W30 is better than 5W30.
3. Changing oil at 3,000 miles is not required with today's technology. 5,000 or even 7,500 mile oil changes are fine as long as your engine is running properly, i.e. no other engine materials contaminating the oil.

One source you can read that will show merit to my statement is More than you wanted to know about engine oil

I had several oil analysis tests ran on the oil from my Toyota Tacoma. At 5,000 miles the oil was still practically new. I'm going to do the same on my Xterra. Others should do some tests on their brand of oil to see which ones are the best. Let me know if you're interested in something like this, I have a buddy who can get us Oil test kits for about $19, including shipping to and from the lab. E-mail me at james.orrand@comcast.net as this forum will not notify me of new posts to this message.