Zapata,
One thing you might want to try, since you noticed a difference with different oil, it to try an oil with a higher ppm of molybdenum...something like Pennzoil high mileage formula or RedLine.
But, you will notice that the owners manual states that if the valves are making noise that they should be adjusted. Our Hondas call for this service every 30k miles. The XTerra calls for it when "necessary". I think you've reached that point.
Bogatyr