Timaca, for what it's worth, I''m at 7,600 feet elevation, and have cooked lentils and split peas three or four times each -- no foaming issues. I'm generally impatient, so I have always used the quick release.
Do you think the problem is with the machine itself? I don't know what it does to create a seal, but it doesn't sound like that seal is happening adequately, if there is foam. Of course, I don't REALLY know what I'm talking about, but I just think you and I should be having similar experiences at similar elevation.
Does it foam for you even if you cook smaller amounts? I don't handle leftovers very well (this is putting it mildly!), so I have a random small mug in which I measure one portion of split peas, or lentils, or rice before cooking. I haven't tried cooking four times that amount, so I can't swear to you that I would be foam-free in all cases.
Those are my thoughts! Maybe you can call the company? Someone here posted that they are excellent to deal with, and really stand behind their product, replacing it if necessary. And since it's a shame to pointlessly add oil to your meals, when others don't have to, perhaps you could see if they would let you replace yours with a new one? This sounds crazy, but y'know, if you don't ask, you don't get.