there is a lot of difference in the viscosities in both cold and hot state of both oils. any oil to be qualified at SAE 50 rating should have a hot viscosity of about 19 cst, a hot 30 should be at 11 cst.
if you are using a 20W50 oil in your engine which was specced for a 30 hot viscosity, you are essentially running an oil twice as thick - which means loss of engine performance, high shear going on while pumping, loss of volume of oil to engine parts and laggy working of the VVT variator (if equipped)