Full beard and moustache here, working on an even fuller Civil War era one now. I do not trim under my nose and contrary to some other posters, I have found over time that it's actually when my 'stache is trimmed and more stubbly that I have more leak problems; when it's longer it seems to "lay flat" and provide a better seal.
I would not use petrolatum or any other oil-based substance on a silicone mask. However, what I have found to be just about perfect is the
Silicone Mask Seal sold by Dive Right In Scuba.
It's simply a medium-weight silicone paste, but it's just right for sealing a mask on your face with the right amount of "stiction", and it absolutely will not harm your silicone mask skirt. Just a pea-sized dab spread around my 'stache just under my nose - along with the thin layer I always put around the rim of my mask - puts a stop to most of the leaking, and one application lasts 2-4 dives. By then what hasn't washed away naturally comes off quickly with just a little soap & water.
It comes in a small jar that will last easily 50 dives, maybe twice that depending on how much you use, and it's small enough to keep in my mask case. It has no smell whatsoever.
Of course, first you should definitely try on as many masks as possible to find the right fit. With masks, that should be the #1 priority.