Thank you sherco40 for the advice!
The testing goes well...
I've created two TYP files for N.America (and Europe), the first one is with only the Map Background and 61 Draw Orders.
The main problems are in CNNA where there are I had to add nonNT polygons for Lakes (and Rivers and Ocean) because the Background was covering them in various zoom levels (highest detail) in Mapsource.
After that I can see for example all the big US lakes from 3000 km zoom level and lowest detail !!!
In the second TYP I've added the Parks polygons, so that we can see the difference between National, State and Urban parks.
Again there are Garmin's errors, like a Desert National Wildlife Refuge that is designated as Urban Park....
I don't really know if those errors were made by Garmin, or by Here (Navteq)[SPOILER][/SPOILER]
I'm also thinking of adding all the water polygons, since the river polylines appear with a slightly different blue color than Garmin's default color for rivers, lakes and sea polygons.