Yes, the potion you speak of grants a +20 bonus to all saving throws. However, i find it a bit redundant -- Potions of Invulnerability stack, and getting your saving throws to below -5 or so is useless anyway.
Yeah, yeah, i know, Greater Malison, Doom, some mosters' attacks cause penalties, yada... I'm generalizing here! 
[img]tongue.gif[/img]
__________________
In memory of Ilan Ramon - the first Israeli astronaut.<br />
<br /><br />[url]\"http://members.chello.nl/~j.vanthull/BG2SR/Home.htm\" target=\"_blank\">BGII Spells Reference</a> - everything you ever wanted to know about spells!