Hi Lodewijk,
this is my oleander
pic was made in end of may, now he is full of flowers, it looks very nice!
I was told that it was the variety 'Hardy Pink'
we had in 2005 50 cm of snow and -13 after that, the oleander was standing in pot but it had no damage. last winter we had 5 days long temperatures below 0 (not lower then -6) and this winter he also had no damage.
if you have a good variety they grow well in our climate.
e.g nerium oleander 'Atlas' is a hardy type.
Troposfeer haves many hardy varieties now (
TROPOSFEER )