Ford Focus>> System of greasing of the engine>> Engine oil>> Engine oil and its classification
Viscosity — this internal friction or resistance to a liquid current. During the winter period of operation it is necessary to apply low-viscous engine oils which provide easier start-up of the engine when ensuring necessary lubricant properties. During the summer period of operation it is preferable to apply more viscous oils.
SAE system (Society of automobile engineers of the USA). In designation of oil two degrees of viscosity are specified: winter with the letter "W" and summer without an alphabetic reference, for example SAE 5W-30. The less first, the is better fluidity of oil at low temperatures. The second is higher, the resistance of oil to high temperatures is higher.
ACEA system (noncommercial Association of automobile engineers). The system operates since 1996 and replaced SSMS system. The class A is applied to petrol engines, for diesel — a class B.
SSMS system (noncommercial Committee of designers of cars). Designation of oil consists of the letter "G" (the petrol engine) or «D» (the diesel engine), and also one number. For example, SSMS G4. The number is higher, the quality of engine oil is better.
API system. At designation of oils are present or the letter "S" (the petrol engine), or "With" (the diesel engine), and also the subsequent letters. For example API-SA and API-CA. The letters alphabetically the following after the letters "S" or "S" are higher, the quality of oil is higher.

Fig. 167. "Ford" recommends engine oil viscosity of SAE 5W-30

Ford for Focus cars with petrol engines recommends SAE 5W-30 engine oil, and also ACEA A1/B1, WSS-M2C912-A1 and WSS-M2C913. For diesel according to WSS-M2C913-A or WSS-M2C912-A. Do not apply some oil to petrol engines with API SC, SD, SE or SF designations (API CC diesel engine), they can damage to the engine (fig. 167) .
Try not to mix oils of different producers, especially at all do not mix the oil intended for the petrol engine, with oil for the diesel.