I dimly recall that a major difference between the 68040 and 68030 MMU is that the 68040 can only handle two page sizes (4 and 8 KB), while the 68030 is much more versatile with around eight or ten. The instruction set to handle the MMU is, from what I remember, more or less the same. Someone with better memory and access to Motorola reference manuals can probably explain things a lot better.