So what does
the owners manual say on cars with the 2.5? If it does indeed suggest premium, then I would guess that your base timing curve is set for premium and using 87 causes it to retard your timing (with a noticable stumble on acceleration).
edit: I guess I'm making up things in my head about the reccomending premium thing... everything I found suggests regular. I guess that if your car has a knock sensor it might be advancing timing a little