Well.........maybe you'd better take the side cover off and have a look in there. Might be good preventive medicine?
If it were MY bike, I wouldn't be worried about 5 minutes of a 20% kerosene mix........but you probably ought to err on the side of caution.
If it takes 30 weight oil or 20 weight, you could probably free it up by putting half a quart of Dexron III or Type A tranny fluid in there with the new oil and running it several minutes.
You put the proper amount in there and it didn't come up to the level check screw on the side of the tranny? 400 cc is about 13.5 ounces isn't it? You sure you're looking at the correct screw hole?
I don't know Garellis very well.......I've been wanting to pick up a shop manual for them.
Yes......carbs can clog up easily when a ped has sat for a while. Clean it out once more and THIS time install a see-thru paper-pleated inline filter on the bike. Does your gas cap vent okay?
Stay in touch! don-ohio