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My car (a 2008 Scion tC) says to have its oil changed every 5K miles or 5 months. I just took a new job and commute into the city by bus so I'm driving a whole lot less. I was told that if I got full synthetic oil on my next oil change, I could simply wait for 5K miles before my next change and forget about the whole 5 months. Is this true?

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The synthetic oil should last longer without degrading, so if you are doing less miles this should safely extend the oil change period.

Check with your garage that you get the correct synthetic oil.

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That's very true, but I still can't get totally comfortable with that as a recommendation. While the oil may not degrade, it's still holding the accumulated contaminants. I'd be very hesitant to recommend exceeding the factory intervals if the vehicle is still under warranty. If you're outside warranty anyway and don't mind taking the chance, well, that's another matter entirely. In my own vehicles I change every 3000 miles with full synthetic, but I don't worry about the elapsed time at all (which can vary from a month to a couple years). –  Brian Knoblauch May 18 '12 at 11:59
To be fair, though, If he flushes and uses synthetic on next oil change, 5000 miles will still just produce 5000 miles worth of contaminants. I'd be pretty happy with that, as long as the 5000 miles doesn't end up taking years :-) –  Rory Alsop May 18 '12 at 12:46

I think it really depends. If you live in an area that can get into lower temperatures (way below freezing) it'd be wise to get lighter oil put in by winter time, and thicker oil by summer time. If the climate is relatively stable this isn't as much of a concern. This is a case where, not necessarily 5 months, but bi-yearly would be a good idea.

When you do end up driving your car be sure it can get up to normal operating temperatures. Otherwise the water (and whatever else) that has collected with the oil in't removed and can lead to oil sludge.

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