Oil appearing in radiator, 4afe engine, is there awater gallery behind oil pump? checked head gasket looked fine

Since there is way more pressure from the oil pump than there is pressure in the cooling system oil can leak into the cooling system and coolant not leak back due to the pressure differential.It is very easy to have a head gasket leaking and not see where the leak was with a visual inspection after disassembly. I would inspect the head for cracks and if it is ok clean the block and head and have the head checked for warpage and if it is ok replace the head gasket and you should be good to go.

Still can be a cracked head gasket. Unless you are creating to much pressure in your engine. Just because you do not see any oil leaks does not mean that the head gasket could not be cracked

The 16 Valve engines have a tendency at high RPM to top up the head with oil - on the earlier engines this can result in oil getting into the intake manifold via the cam cover breather and turning the motor into a 2-stroke. Some cars also have oil surge in this state with attendant drop off in oil pressure. I stress that this tends to occur in race applications only ( or those with a heavy foot in general and enjoy seeing the red line ). This can still occur with the 'Red Top' motor, despite the external oil drain. Most people solve the problem by fitting an external catch tank with a drain back to the sump, with the intake breather taking suction from this catch tank.
Now seeing as how it will load the intake u0p with oil it is likely that if your head gasket is OK then your intake gasket could be shot and oil is being introduced there and showing up in the rad.

Any External location where Oil can be introduced to Coolant should be checked first and replace gaskets etc. as needed. also if Auto Trans Check Radiator foe Trans Cooler Leak. THEN RESORT to Worst Case. Cooling System Pressure Tester Can Tell You If you Are Loosing Coolant.