Thanks for all advices. I`ll run some fuel system cleaner for sure. I don`t think SeaFoam is available here, but I`ll get something considered as good. Thanks for the info about the oil in the air filters, I was seriously concerned that it is something comming from the carbs.
The lowest fuel octane here is A95 so I have to drive with it, wanting or not.
About the speed - this is my first season on the saddle and I`m trying to drive safe - smooth and not fast, with an average speed of around 50-60 mph, so this is not the cause.
However, I don`t have the knowledge to disassemble the carbs and clean them or do any repair, so if there is no serious effect from the fuel system cleaner, I`ll put the bike into the service box and will ask them to chek it. After all, it may need some carb balancing, syncronization or repair.
Thanks again for the advices.