Engine oil change intervals depends on many factors:engine design, its age, working conditions, quality of fuel and engine oil quality. Car manufacturers recommended level of quality engine oil, viscosity grade and periods of replacement. The usual the engine oil change periods  in modern cars, vans and SUVs, with CI engines are 15000 -50000 km.

Thanks Pero,but this range seems quite large. How about the recommendations for vehicles working in tropical dusty areas of the world where ambient temperatures may reach 47oC?

I would like to add one more points that there are  lube oil testing laboratories which test the flash point, viscosity, moisture, TBN number etc . of used lube oil In industries and in locomotive diesel engine they recommend about changing of lube oil or to add some new lube oil to the old one or to change the lube oil+ Overhauling of the engine whether it would be minor overhauling or major over hauling.

The period between changing oil operations at combustion engines depends on a lot of factors. The factors are driving technique, conditions of engine work, atmospheric conditions (dust, contamination and temperature especially), dynamics of accelerating changes (turbo), kind of oil and oil additives, and of course time of oil using. Please forgive me if I have not mentioned all of the factors.So, when an engine oil should be changed? I've researched a lot of oils and machines and if you want to take care for an engine and minimalised its wear, the oil changes should be done between 12 - 15 kkm or every year (at normal using of small and middle engines: road - city). If the air and driving conditions are harder the oil change should be earlier. If you look at the engine durability and wear more short-sightedly you may changed the oil every 30.000 - 50.000 km (or 3 years) but wear of engine will be faster, so the overhaul will come faster.

Thank you Emil. What are the properties you consider in examining the used oil beside viscosity  and carbon residue so as to know if oil has deteriorated.

Dear Kamal, mentioned oil change intervals are very wide because of the influence of many factors on them. For each specific case it is possible to more precisely define the period of oil change, but  You need to use laboratory tests. Can be monitored in service many quality parameters examining oil samples in the laboratory. The main parameters are:  total base number (TBN)-reduce, neutralization number-increase, content of soot in oil-increase, the content of wear metals in the oil-increase, FTIR spectroscopy (oxidation, nitration depletion of additives,....). For example: when the TBN is reduced to half the initial value of the new oil-you must change the oil, or when it comes to crossing the curve of growth neutralization number and curve of decrease TBN, when plotted on the same graph. Some the engine manufacturers recommend limiting values of certain parameters (e.g. content of wear metals, or soot content,...) when are achieved oil must be changed. 

I must agree with Pero. The TBN, content of soot and metalic wear products are main parameters to assess the state of oil. The spectroscopy research give us additional information about additives and contamination.

As mentioned above Oil change oil is dependent many factors, but the major factors are the TBN( Total base number) and amount of sulfur in diesel fuel used. if the sulfur content in the fuel is high it will result to a large amount of sulfuric acid as a by product of combustion. This acid dilutes the base in the lubricating oil until the oil pH became acidic with a catastrophic engine wear. It is therefore necessary to monitor the oil pH to determine at what time to change lubricating oil. It is therefore necessary to do scheduled oil sampling in making an informed decision on oil change. Many oil companies have facilities for carrying out Scheduled oil sampling at a cost but it is an objective tool in determining the health of your Engine.