Remember ONLY top of when it says +1. The electronic gauge is a measure of 1 liter of oil and NOT of the entire oil pan. In my 335 I only had to add once between an oil change and that was due to a slight OFHG leak. At this time ALL my gaskets have been replaced and I never have to top off between changes. Although I do change my oil ever 6k miles.
IF you are using this interval and have the need to add a liter of oil then you might have a leak some where..
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