Well, the reason for doing an oil change in 3K instead of 5K, and change spark plugs earlier, is because ALL of us have different driving habits. For example, if you drive your car hard, it will obviously cause a greater wear in your car. A lot of daily drivers do not know that they drive in SEVERE driving conditions.
One more thing, on older cars (and new cars as well), it is recommeded to change your TIMING BELT at 60K. But if you can, it is SAFER to change it, maybe at 55K if you can. If it breaks, you will be paying for a new engine, which cost a lot more than changing your timing belt. Here again, it depends on the way you drive. YOU TAKE CARE OF YOUR CAR, AND IT WILL TAKE CARE OF YOU. PREVENTIVE MAINTENANCE!
I can tell you this, because I live in Las Vegas. City driving, stop-n-go, extreme conditions. It's like driving a taxi.