a 6x6 is basically a large version of a 4x4 imo, its rather boring in my opinion
I really like my 7x7, even though it is basically a large 3x3 becuase it takes longer to solve so is more satisfying
I just got an 11x11 and i feel great when i solve it, even if you do reduce it to a 3x3 in solving
They are both Silicone lubricants, the difference is the viscosity. Low viscosity oil is for shock absorbers on RC cars - Shock oil. High viscosity oil is for differentials on RC cards - diff oil.
No difference apart from the viscosity.