Motor oil and water

My local auto shop has been trying to drum up business by calling clients and trying to get them to come in for oil changes. They called me up and said it was time for my car’s oil change. I told them it hadn’t even been 3,000 miles yet so I didn’t want to change the oil. The repair guy said, “you realize that even while your car is sitting in the driveway, the oil accumulates water from condensation, so it needs to be changed even if you haven’t been driving the car much.”

I call Balderdash on this line of reasoning. The oil was changed less than six months ago and I don’t believe that there’s enough condensation to make any difference.

What’s the truth?

Boat payment… however you can check for yourself…check the oil dip stick…

As long as you drive the car enough to get it up to operating temperature for a while–eg. a highway run of 15 miles or more, condensation, unburnt fuel, and other volatiles are ‘cooked off’ If all you do are short trips where the car never warms up, then he might be right.

But mostly it’s just BS to drum up business. I would consider changing shops if they persist in trying to sell you unneeded services. Next they’ll be telling you your fuel injection needs cleaned.

If the environmental conditions are humid and most of your driving is short hop, stop and go then what they are telling you could very well be correct.

If the above is true going 6 months between oil changes can be way too long. In some extreme cases changing the oil every 2k miles/3 or 4 months may be necessary.

What they are saying is that your engine can very well be the equivalent of a sweating window. You shut the engine off while it’s hot and heat attracts moisture. Driving the car for longer distances may burn this off; continual short hops will not.
If you’ve ever read about engine sludging then you now know the cause of that particular problem.


First of all, oil does not attract, or even mix with, water. And condensation has to come from somewhere. Condensation is actually the water carried by air. When moisture laden air comes into contact with a cooler surface, the layer immediately in contact with the cooler surface chills and can no longer hold its moisture. It deposits the moisture on the surface. Cold air cannot hold as much moisture as warm air can.

When your car is sitting stagnant in the driveway, the air in the oil pan may have moisture in it, as it was parked in a warm state. When the pan cools, and the air cools, the moisture will deposit on the metal, and if sufficient may even condense and drip down below the oil (oil floats). But, once all the temperatures stabilize, nothing further happens. The oil in the pan does not attract into the crankcase any more moisture than already exist there.

The common feeling on lots of short drives is that the air in the engine never gets warm enough to absorb the water and carry it out, and that the exhaust system never gets warm enough to stop the condensation within. In truth, on really cold days exhaust systems can stay too cold to allow condensation not to happen. Many mufflers even have “weep holes” by a bottom seam to allow the water to drain out.

So, while if your daily drives are all short trips you should follow the “severe use” maintenance schedule for your engine, which is probably no less than 3,000 miles, your engine does not accumulate moisture just sitting.

The call was an attempt to generate revenue. It dismays me that a story like that would be used. But, in truth, there are a many out there who really don’t understand what condensation is and where it comes from. Perhaps the caller was one.

On low mileage cars I think a spring and fall change in spite of mileage is a minimum.

OK it has been less than 3,000 miles. That’s good, but how many months? – I see 6 months. Different cars have different needs. You should look at the owner’s manual and it should tell you how often the oil should be changed. Follow that advice.

As for your local auto shop, they are looking for business and your best interest and theirs don’t always match. It appears this is one of those times when they don’t match. I would start looking for a new shop.

As a follow-up, this car doesn’t get driven a lot in terms of total miles, but when it does hit the road, it’s for 30 minutes or more at a time on the highway. No off-road, no towing, just into town for groceries and to see friends.

In that case you should be fine sticking with the regular service schedule.

One thing you do need to be aware of is time limitations. For example, your owner’s manual may say to change the serpentine belt, or the timing belt, every 60,000 miles OR 60 months. That means whichever comes first.

Tell him that if he can do it over the phone you will then phone him the money.