The object of a lubricant is to keep two pieces of metal from touching; it must therefore be able to get between them, and must stay there. If the pieces are large and heavy, there will be much greater pressure on the oil than if they are small and light, and the oil must be able to withstand this pressure and resist being squeezed out. The oil that would keep the small, light pieces apart might not be able to stand the pressure of a greater weight, and might be squeezed out from between two heavy pieces. Oil has a tendency to cling to whatever it touches, and thick oil or grease has more of this tendency than a thin, or “runny” oil. If a thick oil or grease is used on light machinery, such as a sewing machine, this clinging tendency would make the machine run hard, and might even prevent its operation. When oil is heated, it becomes thinner, or more “runny.” Through this, an oil used in a hot place might get so thin that it would not lubricate; and on the other hand, an oil that works all right in the heat of summer might get so thick on a cold winter day as to be useless. A slow-moving part of a machine uses a thick oil or a grease; a thin oil must be used for a part that moves at high speed. Some of the parts of a tractor move slowly and some at high speed; some are cool and some are hot. Different kinds of lubricants are therefore required, and it is a grave mistake to use a lubricant that is not suitable to the work that it is required to do. The engine is the most difficult part of a tractor to lubricate, and the part that suffers most if the supply fails or if the wrong kind of lubricant is used. In the first place, it is so hot that any oil will burn, being turned to carbon; the best that can be expected of an oil is that it will resist burning until it has done its work of lubricating the piston and cylinder. A tractor engine is more difficult to oil than an automobile or truck engine for the reason that it works harder and more steadily. An automobile engine is rarely driven to the limit of its power; it has frequent opportunities to cool when running down hill. A tractor engine, on the other hand, works at its full power all day long with no opportunities to cool off. An oil that gives good satisfaction on an automobile might ruin a tractor engine through its inability to withstand the greater heat. The makers of tractors understand the importance of using proper oils, and recommend certain brands and grades; these recommendations should be followed in order to get the best possible results. All makers specify at least two kinds of lubricants, and most of them three; one specifies six, which range from a light sewing machine oil to a grease so thick that it is nearly solid. Whatever the recommendations may be, they should be followed.