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I have always thought that running oil too long would damage an engine, and this article seems to support that theory.

When a lubricant degrades, it forms reaction products that become insoluble and corrosive. So too, the original properties of lubricity and dispersancy can become impaired as the lubricant ages and additives deplete. Much has been published about the risks associated with overextended oil drains and the buildup of carbon insolubles from combustion blow-by.

However, surprisingly little has been said about the impact of fine abrasives in a lube oil as it relates to fuel economy over the engine’s life. One can imagine numerous scenarios in which solid abrasives suspended in the oil could diminish optimum energy performance. Below is a list of several scenarios:

•Antiwear Additive Depletion. High soot load of crankcase lubricants has been reported to impair the performance of ZDDP antiwear additives. Some researchers believe that soot and dust particles exhibit polar absorbencies, and as such, can tie-up the AW additive and diminish its ability to control friction in boundary contacts (cam nose, ring/ liner, etc.).
•Combustion Efficiency Losses. Sooner or later, wear from abrasive particles and deposits from carbon and oxide insolubles will interfere with efficient combustion in an engine. Valve train wear (cams, valve guides, etc.) can impact timing and valve movement. Wear of rings, pistons and liners influences volumetric compression efficiency and combustion blow-by resulting in power loss. As has been previously reported in this magazine, particle-induced wear is greatest when the particle sizes are in the same range as the oil film thickness (Figure 2). For diesel and gasoline engines, there are a surprising number of laboratory and field studies that report the need to control particles below ten microns. One such study by GM concluded that, “controlling particles in the 3 to 10 micron range had the greatest impact on wear rates and that engine wear rates correlated directly to the dust concentration levels in the sump.”1
•Frictional Losses. When hard clear- ance-size particles disrupt oil films, including boundary chemical films, increased friction and wear will occur. One researcher reports that 40 to 50 percent of the friction losses of an engine are attributable to the ring/cylinder contacts, with two-thirds of the loss assigned to the upper compression ring.2 It has been documented that there is an extremely high level of sensitivity at the ring-to-cylinder zone of the engine to both oil- and air-borne contaminants. Hence, abrasive wear of the ring/cylinder area of the engine translates directly to increased friction, blow-by, compression losses and reduced fuel economy.
•Viscosity Churning Losses. Wear particles contribute to oxidative thickening of aged oil. High soot load and/or lack of soot dispersancy can also have a large impact on oil viscosity increases. Viscosity-related internal fluid friction not only increases fuel consumption but also generates more heat that can lead to premature degradation of additives and base oil oxidation.
•Stiction Losses. Deposits in the combustion chamber and valve area can lead to restriction movements in rings and valve control. When hard particle contamination agglomerates with soot and sludge to form adherent deposits between valves and guides, a tenacious interference, called stiction, results. Stiction causes power loss. It causes the timing of the port openings and closings to vary, leading to incomplete combustion and risk of backfiring. Advanced phases of this problem can lead to a burned valve seat.

What I take away from this article is a.) keep your oil changed regularly, and b.) change your oil and air filters, too, and c.) the more particulate and junk you have in your dirty oil, the harder your engine must work, and the less efficient it becomes. Pretty obvious stuff, but now you know why.
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Complilates almost everything I told about oil. And there still people against engine oil flush in medium/high mileage engines as if the particulates were their friends.
 

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Right on for posting, that's some solid info!
 

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people against engine oil flush in medium/high mileage engines as if the particulates were their friends.
What they are talking about is actually unrelated to the kind of buildup you find on the valves and stuff. Valves always need to be kept nice and clean.

What they are referring to is the crap that builds up in the seals. This is actually a serious concern for older engines. Sometimes seafoaming your oil results in your motor suddenly leaking like a sieve. I've seen it myself - on the same note though I have seen bikes that really loved the dose of seafoam in the oil.
 

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What they are talking about is actually unrelated to the kind of buildup you find on the valves and stuff. Valves always need to be kept nice and clean.

What they are referring to is the crap that builds up in the seals. This is actually a serious concern for older engines. Sometimes seafoaming your oil results in your motor suddenly leaking like a sieve. I've seen it myself - on the same note though I have seen bikes that really loved the dose of seafoam in the oil.
Yes I know what you mean Spears Masta, my vehicles loved, but I've never let'em to get too much phucked. Don't leta possible leak stop you.

The medium play between bearing parts in a bike engine is about 25 microns to form the oil film, so the particle has to be minor than 20, even tough some filters don't get below 30-40 microns ...


By the way I would risk to go further to point my thinking, never revealed here, that the more of viscosity imply in a wider tolerance to bigger micra contaminants, too. Say a 5W20 will tolerate about 10 micra until it scratches the parts and a 20W50 will prevent up to 20 microns (in spite to having more drag and so be less fuel efficient). A thicker oil will keep the parts more appart, you know? That's not because of compressibility issue, but for outflowing resistance being better in the grossest.
 
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