the toyota oil is mineral i believer, use it till 10k km (oil change should be at 5k), than if you want shift to synthetic, this is what people say:), i used caltex formula (10w30), in a gli 03 model for 180k km (changed every 5k km), and it gave me no problems, synthetic last more but due to our dustier environment i believe you shouldn't use it as advertised i.e. 10k~15k km.
the main problem is getting original oil what ever it may be mineral or synthetic, so get it from a reputable place. secondly the viscosity you chose is fine i believe, and 20w-50 is too thick for current generation engines, though experts here can shed more light on this.