I remember these things being a bit picky with RAM. According to the Service Manual they need 70ns or less 36bit 4MB or 16MB SIMMs. It doesn't say if EDO is compatible so I'd guess not.
Ok, so not really proprietary then, just not the common 32-bit simms that I happen to have laying around. Seems odd that 8mb is not acceptable, even if 36-bit.