I used to be the grease and lube guy for a trucking outfit. They would determine oil change intervals after doing an analysis of the used oil and see how much and what kind of contaminant was in the oil. The bottom line on oil changes is that no matter what kind of oil you use, both the quality of the filter and how often you change it is really more important. Some of the oil quick change shops use very cheap filters knowing that you don't know the difference. (At least WalMart uses a half way decent Framm filter on their changes.) If you drive short distances, cold weather, (rich fuel conditions) dusty conditions, towing, high compression (engine wear), then change your oil frequently (3000 miles is just fine). If you do longer trips in other than the above conditions then 5000 to even 7000 miles is just fine. Just use a good filter that will get rid of most of the particles in the oil. Look on the filter box and see how efficient it is at getting rid of small particles. All good filters will show this information. Remember, no oil filter gets rid of the fluid/fuel that seeps into the oil so an older engine really needs more frequent oil changes. I personally use synthetic because of better flow and starting in cold weather and better protection against wear, but I do keep my vehicles for 10+ years and this is the cheapest "insurance" you can buy. Using the best doesn't really cost that much more especially if you're going to resell a classic or sports car. It's all relative, how long are you going to keep the vehicle and how much wear and tear are you going to put on the engine? If I leased a vehicle, I'd only change it at the required interval with the cheapest stuff available. Also, if you're big on keeping your vehicle in top shape, then keep records and receipts. I'd pay more for and buy a car knowing it's history and knowing that it's been well taken care of. That means engine oil changes, trans oil changes, belts, hoses, antifreeze, brakes, recall items, tire rotations and balancing, etc.