Shed light on this subject

[TransAm12sec]

Increased Maintenance Expenses

When a gasoline or diesel engine idles for prolonged periods, the engine oil becomes contaminated more quickly than when the vehicle is being driven. Oil contamination is more prevalent in diesel engines because of the large amount of intake air used in the diesel combustion cycle. When idling at low rpms, such as 600 rpm, the excess air in the combustion process cools the cylinder liners, resulting in incomplete combustion and condensation of unburned fuel on the cylinder walls. These deposits are eventually drawn into the engine sump where they contaminate the engine oil and reduce its effectiveness as a lubricant.

Controlled studies demonstrate that prolonged idling typically reduces the operating life of engine oil by 75 percent, from 600 engine-hours to 150 engine-hours.

Compare the convenience of excessive idling against the cost of more frequent oil and filter changes and the amount of time your truck will be out of service while the work is being done.

From http://oee.nrcan.gc.ca/publications/infosource/pub/fleetsmart/publications_idling.cfm#happens

So, when it's cold out(or warm), what's the median between letting the car warm up so the oil becomes thin enough to lubricate the engine, but not long enough so the gas doesn't contaminate the oil?

[Prince Valiant]

Well, you ask a leading question...oil doesn't need to be thin to lubricate. It'll lubricate when nice and thick too.

Basically, you want oil to quickly cycle throughout the engine so that all parts are lubricated. In most cases, this takes just a couple of seconds. If the car has sat for a long period, this may take longer.

In the case where significant contamination has taken place, I would imagine that it would take quite some time, but that the time accumulates during every start when the engine is cold, and there is not a significant amount of cylinder pressure to cause the rings to expand with significant tension against the cylinder wall, sealing the combustion process, thus allowing some contamination from the get go. So, probably at ALL times on start up, some contamination is occuring...and this can accumulate.

So, ideally, you really want to let your vehicle idle for as little as possible. Maybe on the coldest days, it would be wise to let the car idle for about a minute...but it shouldn't ever need anymore than that otherwise. Just start and go.

If you are worried about how YOUR driving style might be affecting the life of your oil, you can always get your old oil analyzed....you can tell alot from the results, whether there is active wear, how much, contamination, etc...

[Russ Jerome]

Fleetsmart and Smartdriver programs are aimed at diesel truck
operations. 12ltr trucks do in fact need to keep ther idle up for
a number of reasons not coverd there. lack of throttle plate and
the inj pumps sole source of idle control circut is main reason.

Leave an 11ltr truck at low idle and after an hour the blowdown
tube (not seen on brand new trucks) will look like its own exhaust
pipe, blowby and crankcase pressure will build and gather a huge
puddle of oil under pan. Differnt animal than your computer monitored
solid block (trucks have liners) gasser, let er run all night!