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Message: <blockquote data-quote="ciklee" data-source="post: 638592" data-attributes="member: 22701">Pardon my ignorance but I don't understand what has our climate got to do with engine testing? As far as I know, the engine is a closed loop system and the engineers just need to make sure it behave according to specifications at its normal operating temperature. For most cars, this is around 90-100 deg C. I might be speculating here but manufacturers such as BMW might even test their engines at broader operating temperature range (i.e. between 60-130 deg C, example only bcoz I don't know the actual test environment) to stress the engine and simulate accelerated engine wear. I believe they should have enough test data, with respect to oil sludge problem, to recommend such long service interval. Again, I'm speculating here bcoz i don't have access to their test data.I believe most oil sludge problem occurs when the engine is operating outside its specified temperature. A defective or inefficient cooling and emission control systems probably the one to blame rather than oil change at 20k km intervals. A relatively cold engine will burn rich fuel-to-air mixture, meaning more hydocarbons that can contaminate the oil, accelerating sludge formation. Coupled with a defective EGR system, the engine will surely rosak. Cold running engine could be due to faulty thermostat that is stuck open, allowing the coolant to circulate around the engine and then cooled down by the radiator. Faulty fan that is always running can also contribute to this. An engine that is running too hot can crack the head allowing coolant to get into the engine oil. This would definitely form sludge at lightning speed. A hot running engine can be caused by insufficient coolant due to leaking, or faulty thermostat that is stuck closed. So always check coolant level and identify if it is leaking. If you find yourself needing to top up coolant, then there's a leak somewhere. Another tip is to feel the outlet hose when the engine is cold and at operating temperature. If you do this routine enough, you can tell when the cooling system might need to be looked at. So always check the temperature gauge, both when it is too hot as well as too cold. It should be near operating temperature most times. For E60 guys, too bad the car doesn't come with one but coolant temperature can still be checked from one of the hidden OBD menus. Also, get into the habits of warming the engine up from cold start, especially if you do a lot of short trips.</blockquote>

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