By means of light scattering, static and dynamic critical behavior was investigated for polymer blend solutions consisting of two polymers with highly asymmetric polymeric indices, that is, poly(methyl methacrylate)/poly(dimethyl siloxane)/benzene and poly(methyl methacrylate)/poly(dimethyl siloxane)/toluene solutions. Isothermal susceptibility S-T and correlation length xi in the benzene solution showed Fisher's renormalized Ising behavior characteristic to ternary mixtures in the very limited region near the stability limit. On the contrary the toluene solution exhibited the renormalized behavior over the wide temperature range, reflecting asymmetric phase behavior arising from solvent selectivity of toluene. The temperature dependence of the critical-fluctuation decay rate Gamma for both solutions exhibited "quasihydrodynamic" behavior far from the stability limit. which became the macroscopic hydrodynamic behavior with temperature approaching the critical point. We also measured the tracer-diffusion coefficient D-t of constituent polymers by pulsed-field-gradient nuclear magnetic resonance (NMR) to demonstrate that the quasihydrodynamic behavior may come from the dynamic asymmetry, i.e., difference in the translational mobility of Brownian motions between component molecules.