Dear Car Talk:
A friend of mine has claimed that there's no reason to change oil. He basically changes the oil filter, and adds a bit of oil if needed. He claims that the oil doesn't break down, and just needs to have a new oil filter periodically. Your thoughts? -- Mel
Well, the Mechanic's Children's College Fund wholeheartedly endorses your friend's advice, Mel. Because an engine rebuild a month can put a kid through Harvard.
Oil does break down over time. Is motor oil better now than it's ever been? Yes. Is synthetic oil even better? It is. You might be able to go 10,000-15,000 miles (that's a year, for most people) between changes now with synthetic oil, whereas 30 years ago, we recommended changing your oil every 3,000 miles.
But eventually, the molecular structure of the oil does change. Oil is made up of long-chain hydrocarbons that, miraculously, cushion the metal parts of your engine that rub against each other thousands of times a minute. And when those chains break apart, the oil doesn't do nearly as good a job of providing that cushion.
Plus, heat and oxygen combine with the oil to increase acidity and, if left long enough, it forms varnishes and sludge in the engine. And adding an occasional quart of new oil doesn't do anything to improve the old, broken-down, acidic oil that's still swimming around in there. Your buddy may have a quart of oil in his car that's been circulating since the Nixon administration.
And while the filter will trap any large particles (or rocks or small children) that the oil picks up in its circulation, the filter won't do anything to prevent the oil's molecular breakdown.
So if you have a car that you want to keep for a long time, you absolutely should change the oil at the recommended interval.
The other reason your friend's approach is silly, in my opinion, is that it's a messy pain in the neck to remove and change the oil filter. Once you're set up to do that with your ramps, your drain bucket and your wrench, and you've already got oil running down your sleeve, why not take the extra three minutes and drain the oil, too? It's like taking the time to sit on the toilet but refusing to pee. It doesn't make sense, Mel.