I change mine every 3 months or 3000 miles whichever comes first. I have a friend that has 309K miles on a bone stock 91 NSX (he is the original owner) uses Pennzoil and changes it every 3000 miles without fail. He gets his valves adjusted every 15k religiously as well. I saw it with the valve covers off, at his last valve adjustment and it is almost as clean as the day he took delivery of the car. He had a compression and leak down test a few months ago and everything is where it should be and the engine still runs quiet. He's wondering if he should get the engine rebuilt but, our local NSX guru said it could easily go another 80 to 100K. The car looks and runs as well or better than a car with less than 40K on the clock. He doesn't get all caught up in the oil hype that many of us do and the only reason he uses Pennzoil is because the only mechanic that's ever worked on it uses it.
My dad, who is a retired mechanic, has always told me to use the oil recommended for your car and never ever go beyond 3500 miles between changes. Plus, do the maintenance on schedule when it's scheduled and your car will run for 300K or more depending how you drive. By using his advice, I've driven each of the last six cars I've owned for well over 300K before I sold them or donated them still running well. This thread is the first time that I've ever heard of sending your oil to a lab for analysis. But, if you track your car regularly, I guess it makes sense due to the stresses track use can produce on an engine.
