The Blob, except it also vomits up rodents. Let's see how playable it is.
5 ooze RHD are... not good, to say the least, by which I mean 'somewhat worse than aristocrat hit dice'. At least they have a host of useful immunities.
Ability-wise, summoning oozes aren't as bad as some of their mindless cousins. +4/+2/+6/+0/+2/+4 is reasonable for a medium creature, and leaves the ooze without considerable deficiencies. On top of this, they get 60 ft. blindsight, fast healing 5, +7 natural armor, two useful immunities to acid and fire, and a slow climb speed. In terms of natural weapons, summoning oozes have two natural slams that deal unimpressive base damage with some acid tacked on.
The main ability, of course, is Summon Monster. As a standard action, a summoning ooze can conjure an extraplanar critter, further boosted by the ooze's Augment Summoning bonus feat. This ability has an impressive CL of 20, but is limited to summoning twice the ooze's HD in creatures per day. In addition, while it starts off as Summon Monster I, every two HD (that's HD, not RHD) the ooze gets upgrades the spell a little: Summon Monster 2 by 7 HD, SM3 by 9 HD, and so on, until SM6 at 15 HD.
Honestly, this seems like a somewhat difficult monster to rate, mostly because of the scaling abilities. Sacrificing 5 HD to get a few summoned badgers a day isn't too impressive, but getting to summon chaos beasts and bralani for the same cost at ECL 15 is a different matter.
Comparing the summoning ooze to a summon-focused tier 2 or 3 class, however, kind of puts its abilities in perspective, in no small part because of its difficulty finding suitable class levels. A level 15 sorcerer will have a far more considerable summoning capacity than a level 10 sorcerer summoning ooze, even with the latter's free castings of Summon Monster VI (after all, the sorcerer is already up to Summon Monster VII and Planar Binding).
With this in mind, I think the only fitting LA is -0.
DMs willing to experiment a bit could consider removing 2-3 of the RHD (depending on balance point). This way, a caster is still a spell level ahead, but the ooze's abilities are both less costly and more useful at the levels they become available.