Specific Gravity Specific gravity is a comparison of the weight of the  substance to the weight of an equal volume of distilled water  at a specified temperature. As an example, water weighs  approximately 8 pounds to the gallon; oil with a specific  gravity of 0.9 would weigh 7.2 pounds to the gallon. In the early years, the performance of aircraft piston engines  was such that they could be lubricated satisfactorily by means  of straight mineral oils, blended from specially selected  petroleum base stocks. Oil grades 65, 80, 100, and 120 are  straight mineral oils blended from selected high-viscosity  index base oils. These oils do not contain any additives except  for very small amounts of pour point depressant, which helps  improve fluidity at very low temperatures, and an antioxidant.  This type of oil is used during the break-in period of a new  aviation piston engine or those recently overhauled.  Demand for oils with higher degrees of thermal and oxidation  stability necessitated fortifying them with the addition of  small quantities of nonpetroleum materials. The first additives  incorporated in straight mineral piston engine oils were based  on the metallic salts of barium and calcium. In most engines,  the performance of these oils with respect to oxidation and  thermal stability was excellent, but the combustion chambers  of the majority of engines could not tolerate the presence of the  ash deposits derived from these metal-containing additives. To  overcome the disadvantages of harmful combustion chamber  deposits, a nonmetallic (i.e., non-ash forming, polymeric)  additive was developed that was incorporated in blends of  selected mineral oil base stocks. W oils are of the ashless  type and are still in use. The ashless dispersant grades contain  additives, one of which has a viscosity stabilizing effect  that removes the tendency of the oil to thin out at high oil  temperatures and thicken at low oil temperatures. The additives in these oils extend operating temperature  range and improve cold engine starting and lubrication of  the engine during the critical warm-up period permitting  flight through wider ranges of climatic changes without the  necessity of changing oil. Semi-synthetic multigrade SAE W15 W50 oil for piston  engines has been in use for some time. Oils W80, W100,  and W120 are ashless dispersant oils specifically developed  for aviation piston engines. They combine nonmetallic  additives with selected high viscosity index base oils to  give exceptional stability, dispersancy, and antifoaming  performance. Dispersancy is the ability of the oil to hold  particles in suspension until they can either be trapped by  the filter or drained at the next oil change. The dispersancy  additive is not a detergent and does not clean previously  formed deposits from the interior of the engine. Some multigrade oil is a blend of synthetic and mineral- based oil semisynthetic, plus a highly effective additive  package, that is added due to concern that fully synthetic oil  may not have the solvency to handle the lead deposits that  result from the use of leaded fuel. As multigrade oil, it offers  the flexibility to lubricate effectively over a wider range of  temperatures than monograde oils. Compared to monograde  oil, multigrade oil provides better cold-start protection and a  stronger lubricant film (higher viscosity) at typical operating  temperatures. The combination of nonmetallic, antiwear  additives and selected high viscosity index mineral and  synthetic base oils give exceptional stability, dispersancy,  and antifoaming performance. Startup can contribute up to  80 percent of normal engine wear due to lack of lubrication  during the start-up cycle. The more easily the oil flows to  the engine’s components at start up, the less wear occurs. The ashless dispersant grades are recommended for aircraft  engines subjected to wide variations of ambient temperature,  particularly the turbocharged series engines that require oil  to activate the various turbo controllers. At temperatures  below 20 °F, preheating of the engine and oil supply tank is  normally required regardless of the type of oil used. Premium, semisynthetic multigrade ashless dispersant oil is  a special blend of a high-quality mineral oil and synthetic  hydrocarbons with an advanced additive package that has  been specifically formulated for multigrade applications.  The ashless antiwear additive provides exceptional wear  protection for wearing surfaces. Many aircraft manufacturers add approved preservative  lubricating oil to protect new engines from rust and corrosion  at the time the aircraft leaves the factory. This preservative oil  should be removed at end of the first 25 hours of operation.  When adding oil during the period when preservative oil is  in the engine, use only aviation grade straight mineral oil or  ashless dispersant oil, as required, of the viscosity desired.  If ashless dispersant oil is used in a new engine, or a newly  overhauled engine, high oil consumption might possibly be  experienced. The additives in some of these ashless dispersant  oils may retard the break in of the piston rings and cylinder  walls. This condition can be avoided by the use of mineral  oil until normal oil consumption is obtained, then change to  the ashless dispersant oil. Mineral oil should also be used  following the replacement of one or more cylinders or until  the oil consumption has stabilized. In all cases, refer to the manufacturers’ information when oil  type or time in service is being considered. 6-4