Which Mobil oil is the best ???

An engine oil's job is primarily to stop all the metal surfaces in your engine from grinding together and tearing themselves apart from friction whilst transferring heat away from the combustion cycle. Engine oil must also be able to hold all the nasty by-products of combustion, such as silica (silicon oxide) and acids in suspension. It cleans the engine of these chemicals and build-ups, and keeps the moving parts coated in oil. Finally, engine oil minimises the exposure to oxygen and thus oxidation at higher temperatures. It does all of these things under tremendous heat and pressure.
Multi grade oils are popular and are rated typically with 2 numbers ie: 15w40 which shows 1) the thinness (viscosity or how runny it is!) at low temperature (W=Winter) and 2) the thickness (viscosity or how sticky it is!) at high temperature (20w50 is thicker and 10w30 is thinner). If the numbers are close together then the operating range and performance is more uniform. The second number is the important one unless you live in subzero climes! Use the recommended oil for you car - older cars sometimes prefer a slightly thicker oil to avoid leaks and burning in the cylinder but consult your workshop manual for your car. Choosing oil that is too thin can cause permanent leaks in the seals. Running low on oil will typically wear the top of the engine and this wear is typified by a rattle in the top of the engine on acceleration especially when cold, the pistons will also wear more quickly and you will start to burn oil (well, your car will) producing a blue exhaust discharge. Do not use a different grade to the recommend one unless the engine is very worn (in which case go thicker) or if the engine has been rebuilt (a thinner oil may be needed but the engine rebuilder will recommend the correct grade.)

Never put a semi or mineral oil in a car that requires fully synthetic unless this is an absolute emergency and get the oil drained and corrected as soon as possible afterwards. Try also to avoid mixing viscosity grades of oil and certainly never mix oil types.