A : Lubricant is a substance interposed between two surfaces in relative motion for the purpose of reducing the friction and wear between them. Lubricant provides a protective film which allows for two touching surfaces to be separated and "smoothed," thus lessening the friction between them and correspondingly less heat generation.

The viscosity of the oil for any application depends on the speed, load, and the clearance between moving parts, type of lubrication and finally the ambient temperatures. Oil must provide protection over a wide range of temperatures from cold starts to long hauls. When oil does well at both extremes it gets a “multi-grade” rating that includes a number/letter followed by W and a number, such as 20W40. In a hot climate or driving through a region where a combination of high heat and high-altitude are encountered, the operation & maintenance manual provided by the manufacturer, will probably give suggestions for a multi-grade or “all-season” oil such as 15W-40. Such oil has the cold flow characteristics of 15-weight at start-up, yet the coating and protecting property of 40-weight oil at the high temperatures. In a predominantly cold climate, the recommended oil might be a 5W-30. However it is advisable to be guided as given in the operation & maintenance manual of that equipment.

The performance levels of chemical properties relating to detergent action and dispersion in the two engine oils are different. Accordingly API service classes have two general classifications: S for gasoline engines and C for diesel engine. It is possible for an oil to conform to both the gasoline and diesel standards.
(Note: Combined API Service Classification motor oils are highly effective at reducing and controlling sludge and varnish deposits, acid and foam formation and have improved oxidation stability. Most of the oils in the SPERRY range meet such minimum levels and therefore possess the required standard and accordingly they are suitable for both petrol and diesel engines.)

The performance enhancing additives of the oil viz. TBN, anti-wear, ant-oxidant etc. gets used up in service and below certain levels the oil will not be in a position to protect the engine. Oxidation of the oil beyond certain limits will start corroding the equipment or leave unwanted deposits on the moving parts. Further contamination of the oil with water, dust, dirt and soot from combustion products shall cause corrosion and abrasive wear. In view of these reasons that we need to change the oil once it looses its useful life to protect the engine.

Good quality Coolants have the special rust preventive pigment which forms a coat over the inner part of the engine. If good Coolants are not used rusting, scaling & sludge formation takes place in the engine, which results in blocking the Coolants path thus overheating the engine & reducing the performance of the vehicle.

Certain amount of oil separates from the grease depending on the operating conditions and also during rest periods or during storage. Excessive separation of oil from the grease during storage indicates that the grease has not been cooked well.
Grease must be removed uniformly from all sides of the pack for application because any cavity formed shall results in the oil separating from the grease in the pack. Pack only half or a third of the bearing housing with grease as cramming the bearing housing with grease can result in overheating, leakage and a possible seizure due to the non-availability of adequate oil.