The color of the oil has no special meaning, but it depends on a number of factors such as the refining process and the types of base oils, additives and crude oil used. The oil might turn a shade darker after use because it contains a number of suspension particles. The reason for this color change is that the oil cleans the engine of particles and other deposits produced by the high temperature combustion. These particles are then kept in suspension to avoid depositing them on the engine’s surface.

Petrol and diesel engines have different lubrication requirements. During development, lubricants undergo a number of tests, some common to diesel and petrol engines and others specific to one or the other type, that categorize their performances. At the end of this process, they are awarded an API or ACEA/CCMC compliance standard. These specifications characterize the acceptable functioning of the lubricant in each type of petrol or diesel engine. Lubricants for four-stroke engines usually comply with both petrol and diesel specifications. For example, an API SJ/CF lubricant meets petrol specification SJ and diesel specification CF. But this does not necessarily mean that it will perform in the same way in both petrol and diesel engines. This is why Octagen has developed a specialized range of lubricants, optimized either for petrol or for diesel engines.