Polymer blends show a much larger 3d-Ising regime, e.g. a much larger Ginzburg number Gi than predicted by the Ginzburg criterion. This discrepancy is supposed to be explained by the compressibility or the free volume of the blend. In this paper we present and discuss the Gi number of polymer blends, different in chemistry, molecular weight, and microstructure. One sample was also studied at a different pressure. Gi was determined by describing the susceptibility obtained from SANS with the crossover function from Belyakov et al. The essential results are that Gi depends sensitively on monomeric microstructure and on pressure. This clearly shows that Gi is not a universal function. The observed strong decrease of Gi with pressure is a clear experimental proof that the critical crossover behaviour in polymer blends is indeed strongly influenced by the compressibility or free volume of the blend.