[M.D. Holloway]

The oil can shear down for a few reasons but none have to do with the oil having fibers in them. That would not be a good thing. Shearing takes place for a few reasons and it doesn’t happen with all oils.

Finished oils (engine, hydraulic, gear, ect…) are a combination of base oil(s) and an additive collection. The additive collection are chemicals that provide performance beyond the base oils properties. These additives provide viscosity stability, anti-foaming, anti-wear, anti-corrosion ect…. The combination, chemistry and concentration of these additives along with the base oil will provide the performance needed for component protection and extended oil life.

One of the additives that are used commonly in engine oils is Viscosity Index (VI) Improvers. VI improvers control the viscosity of multi-grade oils. They are polymers which act like "popcorn". At low temperatures, they are "tight-balls" which do not significantly increase the oils resistance to flow. However, at high temperatures, these "tight balls" explode into long chain polymers, which interweave and increase the oil's resistance to flow (viscosity). The tendency of oil to "thin" at high temperatures is controlled and reduced.

These polymers (long chain molecules) help the oil remain in viscosity but under certain conditions they may cleave or break. When this happens, the viscosity of the oil actually gets reduced at operating temperature. This can spell trouble.

Also, some additives will be introduced using a carrier. This carrier will normally be in a base oil but in some cases because of the nature of dilution some are in a carrier that is high in viscosity. This is done to offset any viscosity swing that can happen upon blending. Remember, an oil blender has to hit the right viscosity.

Some people thing that the base oil shears down. This is partially true yet very difficult. The energy it takes to snap an oil molecule is pretty high. It is done all the time at oil refineries (cracking) yet for it to occur in a crankcase is pretty tough. What does happen is that the oil can cleave due to an increase in the oils acidity. If you have ever smelled oil that is acrid it is because it has become acidic. In the engine, acid formation is very common. Acids such as sulfuric, nitric and carboxylic are very common. These rascals are the one of the main reasons deposits form. The acids actually cleave the oil molecules and the active ends crosslink with other active ends and deposits such as sludge form (with the introduction of a nucleation site like grit or sand particles) and also varnish on moving, dynamic surfaces. If pressure and stress is high enough then lacquer forms. Synthetic base oils or highly refined mineral oils show a resistance to shear or viscosity breakdown - thats a much more technical reason saved for a later discussion.

Now, I have seen mineral oils shear down with ultra-sonics in industry. Thats a whole other story and possible a thesis if I ever want to finish my PhD!

The viscosity of spent oil will go thin at first then get thick. It is all a rate equation with a bunch of independent and dependent variable at play. Only way to really ever know is through oil analysis. Does that help?