The good news is that engine takes 8qt so you prob didn't do much damage.

I'm not aware of any level sensor on that motor but having driven one almost 80k miles i can tell you how to determine when it's low:

Count the seconds it takes to prime the lifters!

When my lifter tick took more than about 1.5 sec i was 1 qt low. Very consistent.

Keep in mind: full is the middle of the bump on the dipstick.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)