YS P280

"At the beginning of each session, you may
declare what potions you have on hand to fill
those slots, or otherwise leave them open. If you
have an open slot and a successful Lore roll or a
fate point to spend, you may later declare that
you coincidentally have an appropriate potion."

What difficulty do various GMs tend to use for this roll? 

My GM said "strength of the potion" but to me, that's always equal to your Lore, so you've got a 50/50 chance of having to spend the FP anyway regardless of your Lore/Potion strength unless you have an aspect to tag for free somewhere (which theoretically you could've used to enhance the strength of the potion, putting you back in a situation where you have a 50/50 chance of failing...).  Having a higher Lore wouldn't actually make you more likely to have the "right" potion on hand, leading you to most of the time have to spend a FP.

I'm not sure where I stand on this one other than my usual default of "diff 3" for anything that isn't stated, just like you'd have to place a Maneuver.