Quote:
Originally Posted by nabenson
This.
If you do not drive your car long distances, letting it fully warm up (20 minutes per trip) then your oil eventually becomes heavily contaminated with water. This water comes from the intake, straight out of the air it is sucking in.
This is the reason for the time limit in addition to a mileage recommendation. If you drive few miles, but always drive for say an hour at a time then you're fine to extend the time, but otherwise I would just change it on the time mark or the mileage mark.
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Uh I don't think that's quite right.... intake air goes into the combustion chamber and back out the exhaust. Precious little water gets in that way. If it was as heavily contaminated as you suggest you'd have milky oil all the time. Typically condensation is where most of the moisture comes from that gets in the oil. If your car sits for extended periods of time you're more likely to have extra water in your oil.
Water from condensation is why you should change your oil if your car is off the road in the winter - simply change the oil in the spring when it goes back on the road. In the winter turn it over every few weeks to keep the seals oiled. Putting oil in the car for storage doesn't make any sense either since you're not going to drive with the "storage" oil so just leave the oil that's in it when you park it.