Olaf,
I'm currently using full synthetic Amsoil HP Marine 2-Stroke Oil. It probably isn't available in your area, but I'm sure there's a similar product. It works great for me with little carbon build-up and no ring sticking over 110 hours. The only problem with full synthetics is they don't provide much protection for storage, so you need to fog your engine if you don't fly often (like the long winter).

Here's a link: http://www.amsoil.com/shop/by-produc...-2-stroke-oil/