It's surprising that as the "how often to change the oil" thread is re-started,
ad infinitum, people seem to only concentrate on using mileage as a metric of when to change.
Usually (as in this particular thread) no mention is given to the BMW Condition Base Service (CBS) system (pp. 219 BMW NA Owner's Manual) that has been engineered into our X3's by the people who designed them.
Surely, anyone can understand that the useful life of a sump full of oil will depend on the use the vehicle gets. Unless you're willing to send samples of your oil to a well respected laboratory like
Blackstone Labs, you really have no idea if you're changing way too soon (and wasting your money and valuable natural resources) or you're risking extra engine wear by driving with oil that's been degraded.
BMW (as well as other manufacturers) has addressed this by incorporated a system of sensors, timers and "special algorithms" to efficiently optimize the oil change intervals. Although the current BMW computational algorithm is not published, its characteristics have been pretty much derived by owners. Apparently it projects the “mileage to the next oil change” by starting with a maximum possible mileage interval and then subtracting “miles” using different weights for miles driven under different conditions. It also records the number of “cold” starts and “warm” starts and uses that data.
Miles driven when the engine oil is cold are given more weight (apparently up to 2.5X), mileage driven at high engine rpms are given more weight, and mileage added when the engine is cold and also revved (apparently above 4500 rpm) is given much more weight. The “weighted” miles are subtracted from the starting number and the result is the projected optimum change point.
On my X3 35i, we’ve been doing a lot of “short-hop” drives and only a few highway trips. Even though I always try to drive long enough to get the oil warmed up, that’s not always the case. Right now our X3 has about 5000 miles on the odometer and the CBS display (visible every time the car is started, or accessible via iDrive, indicates that the oil should be changed in another 2800 miles.
Considering the very high quality of the BMW spec oil and the way my vehicle has been driven, that change (at about 7500 miles) seems perfectly reasonable. However changing oil on a NASCAR racer that's driven 500 miles at 8000 rpm, or changing at 15,000 miles on a salesman's car that only accumulates highway miles in long steady cruises at 1800 rpm, and is only started once a day, also seems perfectly reasonable.
Just my 2-cents.
