First, you are responding to a 15 year old post. The OP hasn't been here in 8 years.
Secondly, have you noticed that even in modern cars, the manufacturer makes no distinction regarding oil change intervals -- between synthetic oils and conventional oils?
Fact is, it's not the brand or type of oil itself that determines the change interval, it's the engine. How much blow-by does that engine design exhibit? And how much does it have now when you've got 150K on it? I don't expect an answer. What is the oil operating temperature? What kind of PCV and EGR systems does it have? What kind of fuel are you using? What kind of driving are you doing? Are you towing? Are you lead-footed?These are things that affect the usable life span of the oil. Expensive oil does not get less contaminated than cheap oil. And even better oil filters only get the chunks -- none get the contaminants out, those are molecule sized.
I guess I have to get on my soapbox again. CHANGE YOUR OIL AT THE MANUFACTURER'S RECOMMENDED INTERVALS. They were not joking in the users manual. The intervals are designed to keep your engine from being exposed to contaminated oil too much/too long..... it's all a compromise. Cleaner oil == longer life. Period.
This is basic stuff. I work for a vehicle manufacturer.