You should be able to see bright, such as sun light through the filter at all points. If you cannot, it should be replaced. If the filter is only slightly dirty, you can try cleaning it by tapping it on a flat surface or by blowing compressed air through it, from the clean side out. You'll find that you need to replace your air filter more often if you frequently drive on dirt roads or live in a dusty area. What has changed is the materials and design of the engine and the temperature at which it operates.
Maintenance is very simple.
Make sure that the coolant is clean and contains adequate anti-freeze. Have the drive belts checked and tightened periodically. Have the anti-freeze changed as recommended in your owner's manual or maintenance schedule and ask that the hoses be checked carefully. Even though a hose looks OK on the outside, it may be cracking and getting weak inside. Finally, keep an eye on your temperature gauge while driving.