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Message: <blockquote data-quote="f8." data-source="post: 135121" data-attributes="member: 923">0W 30 implies it has a weight of 0 grade oil when cold ie during crank up and a weight of 30 grade oil when at operating temperature (which optimally is at 100 degrees C-note where 100 is indicated on M3's oil temp gauge-should be 6 o'clock).so the lower the first digit, the better since it aids rapid lubrication from standstill to crank up.the weight of 30 is ok for most modern engines. overly thin oils becomes a problem in engines with high wear and tear because the main and con rod bearing clearances may have increased due to bearing wear or even journal wear. in those circumstances the stock oil pump will be struggling to maintain oil pressure because there is too much volume leakage out of these loose clearances. consequently if you have an oil pressure gauge you will see low oil pressure, especially as the engine gets hotter since the main caps would've expanded further then compared to the steel crank. and over time, driving hard with low oil pressure will make the wear worse and worse until the engine seizes.but if the engine has been very well maintained or is relatively new, chances are the clearances are still on the tight side of the factory spec to warrant the use of thinner oils without running into oil pressure issue. the plus side of running a thin engine oil is of course more power due to less oil drag and consequently too, better mileage.the down side is as some say the oil level may drop more over the same interval as would for a heavier oil. but as a discerning car owner who regualrly replaces engine oil and checks oil level, i see no issue with this. some race engines have bigger tolerances for high rpm operation and consumes far more oil than a standard road car. thats why they run uprated oil pumps or even a dry sump system-to keep pressures high even with high volume flow. all that means is just more cautious monitoring of oil level and pressure.</blockquote>

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