In addition you do not need oil additives if you buy good oil in the first place...
The supra is a low oil pressure vehicle and you can only increase that so much at a certain point (mentioned above) it kicks the oil back into the pan allowin less to flow into the engine.
Pressure is resistance to flow...a 20W-50's increase in pressure means that it does not flow as well through the bearings or to the top of the motor.
If you would take a moment and read through
Motor Oil I think you will have a better understanding