Re: "synthetic" oil
For any fluid to act as a lubricant, it must first be "polar" enough to wet the moving surfaces. Next, it must have a high resistance to surface boiling and vaporization at the temperatures encountered. Ideally the fluid should have "oiliness", which is difficult to measure but generally requires a rather large molecular structure. Even water can be a good lubricant under the right conditions.<br /><br />Castor oil has other unique properties. It is highly polar and has a great affinity for metal surfaces. It has a flash point of only 445 degrees F, but its fire point is about 840 degrees F! This is very unusual behavior if you consider that polyalkylene glycols flash at about 350-400 degrees F and have a fire point of only about 550 degrees F, or slightly higher. Nearly all of the common synthetics that we use burn in the combustion chamber if you get off too lean. Castor oil does not, because it is busily forming more and more complex polymers as the temperature goes up. Most synthetics boil on the cylinder walls at temperatures slightly above their flash point. The same activity can take place in the wrist pin area, depending on engine design.<br /><br />Castor oil meets these rather simple requirements in an engine, with only one really severe drawback in that it is thermally unstable. This unusual instability is the thing that lets castor oil lubricate at temperatures well beyond those at which most synthetics will work. Castor oil is roughly 87% triglyceride ricinoleic acid, which is unique because there is a double bond in the 9th position and a hydroxyl in the 11th position. As the temperature goes up, it loses one molecule of water and becomes a "drying" oil. Castor oil has excellent storage stability at room temperatures, but it polymerizes rapidly as the temperature goes up. As it polymerizes, it forms ever-heavier "oils" that are rich in esters. These esters do not even begin to decompose until the temperature hits about 650 degrees F. Castor oil forms huge molecular structures at these elevated temperatures - in other words, as the temperature goes up, the castor oil exposed to these temperatures responds by becoming an even better lubricant!<br /><br />Unfortunately, the end byproduct of this process is what we refer to as "varnish." So, you can't have everything.<br /><br />In spite of all this, the synthetics are still excellent lubricants if you know their limitations and work within those limits. Used properly, engine life will be good with either product. Cooked on a lean run, castor oil will win every time. A mix of the two can give the best of both worlds. <br />Like most things in this old life, lubricants are always a compromise of good and bad properties. <br />Synthetics yield a clean engine, while castor oil yields a dirty engine, but at least now you know why! <br /><br />Castor oil would be an excellent oil in a race only engine that was air cooled, and torn down periodically to clean the varnish out, but we are talking outboards hear and it just wouldnt do. Good oil, "yes" suitable for outboards? "no"!<br /><br />Apples to apples! <br /><br />--------------------<br /><br />Lubedude,<br />Someone mentioned castor in another message. If a synthetic/castor blend is used (especially in a water cooled engine), varnish buildup is insignificant...specially if routine decarbonizing is done. <br /><br />The reason to use it is for protection from a waterpump going bad. I've cooked small engines to the point of seizing while running and crackling like bacon in a skillet. After cool there is no damage with castor. With synthetic or pet oil the engines are trashed under these conditions...air cooled or water cooled. Everyone has gauges these days so cooking an engine isn't that common. Neither is running castor. But if anyone wants the ultimate protection from damaging an engine from high heat they have to run castor. Nothing else comes close. <br />--------------------------------------------------------------------------------<br />Edited:<br /><br />OK, This time you said "Blend" Id have to say that is better than a full castor!<br /><br />"Cooked on a lean run, castor oil will win every time. A mix of the two can give the best of both worlds." This is a quote from the above post of mine, so are we arguing the same point? Or did you bother to read it all? <br /><br />If you run a full castor oil in an outboard, because of the low heat involved, you are going to have the oiliest mess of unburned oil you ever saw, not to mention an overly dirty engine, fowled plugs and all, JUST IN CASE YOUR ENGINE OVERHEATS?<br /><br />Now I know you do not use castor oil Blend in your outboard yourself and full synthetics are far superior to conventional if overheating occurs without all the bad side effects, and there arent any castor oil blends specifically designed for ourboard use that I know of. The Amsoil 2 cycle oil contains a certain amout of esters anyway which is what castor oil turns into when overheated.
