Per the porn ive read, i believe the word with the u is preferred for both noun and verb. And multiple m's are used in the gerund form. Pretty much just repeat any letter in the word multiple times, no pun intended.

Eta - followed by lots of exclamation points!!!!

I guess consistency isnt a hallmark of that sort of writing, else you could have gleaned it from the usage?

Eta - my bad - they usually cover consistency in the following paragraphs...