Dear EarthTalk: How often do I really need to change my car’s oil? Conventional wisdom has always put it at every 3,000 miles to prevent engine wear, but isn’t changing oil that frequently wasteful and unnecessary? Also, what is the "greenest" and longest-lasting oil I should use? -- Vic Roberts, Lincoln, MA

There is much debate in the automotive world over how often drivers of typical passenger cars or light trucks should change their oil. The quick-lube chains usually recommend it be done every three months or 3,000 miles, but many mechanics would tell you that such frequent changes are overkill. Indeed, most car owner’s manuals recommend changing out the oil less frequently, usually after 5,000 or 7,500 miles...