Q: How often should I change the oil?

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solamour - 29 Mar 2006 07:37 GMT

Hello, everyone.

The subject says it all. Most people around me say I need to change
the oil every 3,000 miles (or every 3 months). I drive only short
distance, so it takes me about 6 months to reach 3,000 miles. Do I
still need to change the oil every 3 months even if I drove, say,
1,500 miles? Would the oil go "bad" if I wait for 6 months? Any
suggestions welcome.
__
sol

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laocmo - 29 Mar 2006 15:27 GMT

I have a friend who is in the auto business. For what it's worth he
says the danger involved in short trips, especially in big heavy iron
engine cars is that the engine does not get hot enough to drive all the
moisture out of the oil and exhaust system. This can result in sludge
and rusted exhausts. My MX-5 Speed temperature gauge reaches normal
very quickly, a couple of city blocks, compared to my big car, about 4
miles. I assume the temperature gauge is an indication that all the
moisture has been driven out? I'm not sure if the experts would agree.
I'm in the same predicament; I drive mine only when the weather is
nice. I'll probably not get to use up my free years worth of oil
changes at the rate I'm accumulating miles.

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Lanny Chambers - 29 Mar 2006 15:55 GMT

> My MX-5 Speed temperature gauge reaches normal
> very quickly, a couple of city blocks, compared to my big car, about 4
> miles. I assume the temperature gauge is an indication that all the
> moisture has been driven out?

No. Oil temperature is a better indication, as indicated by the oil
pressure dropping to about 60 psi at 3000 rpm as it thins from heat. But
NBs don't come with a real oil pressure gauge, so that won't help you.
At a guess, 5 miles of traffic should get the oil hot enough to cook off
the water.

To answer the original question: short trips qualify as "severe service"
or whatever the owner's manual terms it, calling for more frequent oil
changes. It's not that the oil itself goes bad, but that it accumulates
acids that etch bearing surfaces, and gets diluted with fuel washed down
cold cylinder walls. Fully-warmed engines remove much of this stuff as a
matter of course, but short trips are tough on oil.

Signature

Lanny Chambers
'94C, St. Louis
http://www.hummingbirds.net/alignment.html

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XS11E - 29 Mar 2006 22:41 GMT

> At a guess, 5 miles of traffic should get the oil hot enough to
> cook off the water.

The general rule of thumb is the engine needs to run 30 minutes, 15
minutes to reach operating temperature and 15 minutes running at
operating temperature to boil off all the condensates.

Like most general rules it will vary but how many of us are able to
measure the oil temperature? Since most of us don't know the oil
temperature the 30 minute rule is about as good as any to decide if the
vehicle is subject to "severe" service.

FWIW, GM once allowed an extended oil change interval for "normal"
service and a 3,000 mile change interval for "severe" service. They
found more than 90% were in the "severe" service catagory so that's the
one to follow.

To answer the OP's question, use the owner's manual recommendations.

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Zog The Undeniable - 29 Mar 2006 18:44 GMT

> Hello, everyone.
>
[quoted text clipped - 4 lines]
> 1,500 miles? Would the oil go "bad" if I wait for 6 months? Any
> suggestions welcome.

Only in the US, where everything is cheap, do people do the 3,000 mile
change. In the UK it's more like annually.

Personally I've been convinced by an argument that says you should
change the oil in a daily driver every six months regardless of mileage.
The theory is that if you do a lot of miles in that time, the engine
always warms up well so the oil won't get contaminated and will
deteriorate slowly through shear and heat. If you do few miles, they
will be short trips which allow acids and condensation to build up in
the oil, so you need to change it even though it's not degraded by shear
and heat.

I fall into the latter category because it's only 5 miles to work. Far
enough for the thermostat to open even in winter, but probably not quite
far enough to get the oil really hot.

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