Two points:(big snip) A 5w-40 approaches 16cS which is too thick for the needs of the FJ at operating temps.
1. It's not clear why you are using the term "flow rate"; the lubricant characteristic is "kinematic viscosity". The unit of measure is the Stoke; for typical engine oils, centistokes (Cs) is a more useful unit. "Flow rate" relative to an engine's lubrication system is based on oil pump volumetric delivery, the pressure drop (resistance to flow) through the engine's oil galleries and across plain (rod and main) bearings, and the kinematic viscosity of the lubricant.
2. Can your provide any information from any source that would support your assertion that 5W-40 is "too thick" for use in a 1GR-FE engine at normal operating temperature?