BMW would like us to keep our oil level between about 5.0 liters and 6.5 liters and they would like us not to obsess about the exact level. They would prefer that we didn’t exceed 6.5 liters. To this end my ’08 135i and my ’11 328i both incorporated similar oil level display logic. An acceptable oil level was shown with a bar graph showing the oil level mid-way between min and max hash marks with the text “OK”. In the consumer’s brain this looks like half a liter down from full and “OK” is not a glowing endorsement. Wouldn’t full be better? Those who harbour such thoughts will top their oil up until it eventually tells them it is overfilled.
Somewhere along the way BMW updated the display logic and now it shows “OK” with a full bar graph if the oil level is acceptable, “+1” if it needs a liter to stay within the preferred range, and exceeding 6.5 liters doesn’t produce an overfilled indication. The full bar graph works well for those with OCD.
This system doesn’t work well at the track, where both the N54 and N55 struggle with low pressure transients if the oil isn’t actually full (6.5 liters) or slightly above. The problem is that the oil level management system will never tell us when the oil level is actually full. Unless we have just changed our oil and accurately measured the amount added, we never actually know the oil level. This was brought home to me a while back when I experienced a 4 psi low-pressure transient on track. Typically I see the oil pressure become unstable at 1.0 g with transients as low as 20 psi, but in this case the transient started earlier and went alarmingly lower. Luckily, I was monitoring engine health data on my first session and noted it.
Adding half a liter of oil resolved the problem, but of course the oil level indicator showed “OK” both before and after the addition. I added another half liter just to see if the indicator was working, expecting an overfull indication, but as I discovered the system didn’t show that.
To understand the system, I did some experimenting back at the shop:
4 liters is 2.5 liters low, but is “+1”:
6.5 liters is nominally full and shows as full and “OK”:
8.0 liters is 1.5 liters over full, but also shows as full and “OK”:
My practice is to add 7.0 liters when changing oil on my track car, with the intent of maintaining the oil level between 6.5 and 7.0 liters.
As it happens, the oil level sensor does actually measure the oil level (height in mm) in the oil sump, and this can be viewed as live data with a code reader. I have used both PROTOOL and a Foxwell NT510 for the purpose. A generic OBDII scanner without BMW-specific software won’t work.
Wanting to a) see if my sensor was actually working properly, and b) understand the relationship between the height reading and the oil volume, I undertook a calibration of the sensor by adding oil in increments and taking height readings at each volume.
From this I was able to divine the oil level management system logic, but more importantly to discover that live data provides me with a real functioning “dipstick”. The sensor maxes out at 74 mm which corresponds to about 6.8 liters (or above). Any reading above 70mm satisfies my desired oil level, and the calibration curve suggests the following oil fill logic:
