By Jim Fitch
Practicing Oil Analysis Magazine
There has been much said in recent years about the increased propensity of some modern oil formulations to produce sludge and varnish. Ironically, many of the oils accused of contributing to varnish are the same oils endowed with exceptionally high oxidation stability due to advanced refining technology and formulation science. To better understand this paradox, let’s begin by defining a new term.Lubricants have an intrinsic Impurity- holding Capacity (IHC) relating to their ability to retain very small (submicron) impurities in solution. These impurities are largely oil-degradation products, but include other oil suspensions as well.
This holding capacity is like an imaginary silo or cup within the oil. If the cup is half empty, it has unutilized or reserve capacity. However, conditions can occur that can cause an overfill, leading to insoluble suspensions in the oil and varnish (sometimes called the “dumping point”).