Ask a crowd of farmers to name the worst engine-killing contaminant to get into diesel oil, and they would likely respond in unison, “Dirt!”
Not that dirt or (to be more specific) particle contamination isn't a threat. Better filtration systems on engines, oil storage, and maintenance practices have minimized the threat of dirt contamination.
“It is worth noting that problems are more pronounced when contamination combos exist,” says Jim Fitch of Noria Corporation, publisher of Machinery Lubrication magazine.
An example of this would be dirt particles combining with water to form miniature dirt balls that can greatly reduce oil flow to critical parts.
Following are Fitch's top four most lethal contaminants. He warns there are no additives in engine oil that control or minimize these contaminants.
1. Water. The most destructive of all contaminants is water since it attacks engine-protecting additives in oil, deteriorates oil, interferes with the creation of lubricating film on parts, and boosts the corrosive potential of common acids found in oil.
Water in oil can emulsify, picking up dead oil additives, soot, oxidation, and sludge to create globular pools that can knock out filters and restrict oil flow to bearings, pistons, and the valve deck.
Beyond contamination from poor storage of oil (upright barrels exposed to the weather), a leading cause of contamination is long idling in winter, which causes condensation in the crankcase.
Low levels of water contamination are normal in engine oils. But high levels of water showing up in an oil sample analysis is a red flag that your engine has a serious problem that won't be cured just by changing oil.
2. Glycol (from coolant) entering an engine via defective seals, blown head gaskets, cracked cylinder heads, and corrosion damage is a far more common contaminant than many farmers believe.
What makes glycol such a killer is that it reacts with oil additives to reduces the oil's ability to protect engine parts. And less than 1% glycol contamination is enough to coagulate soot and cause a dump-out condition leading to sludge that restricts oil flow and blocks filters, Fitch warns. In worst-case situations, glycol contamination has led to cold seizure of engines.
3. Fuel mixing with, or diluting in, oil not only occurs but also can get bad enough to drop oil viscosity from, for example, 15W40 down to 5W20 in a short time, accelerating piston ring, liner, and crankcase bearing wear. In severe contamination cases, fuel dilution thins the concentration of oil additives, weakening their protective effectiveness.
You can blame excessive idling, running in cold weather, and frequent starting and stop for causing fuel dilution in engines that are well maintained and in top operating condition. Faulty injectors doling out too much fuel or doing a poor job distributing fuel within cylinders can lead to what is called wash down of oil on liners, which accelerates ring, piston, and cylinder wear.