@contrex,
Learn something new everyday. It was weird- I looked at that word and thought, 'That looks different' - but I wasn't sure- not having used the word in a while and being somewhat prone to misspellings, that I didn't have it wrong so I looked it up in my (admittedly) American dictionary.
But that's very interesting - given that 'traveller' with two l's instead of one is the British spelling and traveler with one l instead of two is the American spelling - and yet with the word skillful - it's opposite - two l's for Americans and one l for the Brits- huh!
But I still think skilful (or skillful) fellatio is grammatically incorrect, much as someone might like to be able to experience the skill well applied without need of s skil(l)ful applier.