Oh man - this is a hard one to describe ... umm - you can feel the pulsing / spasming (call it what you will, each guy has his own little rythm) and there is a warm sensation - a sensation of extra wetness. I wouldn't say 'gooey' - gooey, to me, is like cookie dough or something - I'd say oily. Hey - here's an idea - squirt some babyoil up your ass, warm it to body temperature first of course. Maybe yoghurt would be better... that might give you an idea..
Sorry - re-reading that doesn't sound too helpful