Hello, Jochen
For this, the maps must contain within themselves such data of contour lines (10 m). OAM, lomaps contain only contour lines of 20 m.
The way out of this situation is to completely make such maps or a map containing only the contours of the heights, the required frequency and overlay it with the corresponding theme.
But, again - do it yourself.
Well, or ask map manufacturers to make contour lines of heights more often.