Your engine (see attached) has an exhaust manifold in front of the engine and another in back of the engine, one 3-port manifold for each “bank” of cylinders. Each gets very hot, hundreds of degrees.
You also have a valvetrain over each “bank”, and a valvecover and valvecover gasket on each cover.
As en engine ages, the valvecover gasket loses its “compression” between the valvecover and the head. It can, and often does, weep oil. In addition, older engines like yours develop “blowby” from normal wear. That’s a condition wherein the rings don’t seal as tight as they did when new, due to wear and metal fateigue. This, combustion gasses can get forced by the pistins. This can causes excess pressure in the crankcase. This excess pressure also pressurizes the spaces under the valvecovers, as they’re open to the crankcase through the oil return channels.
This excess pressure can force oil past tired, weepy valvecover gaskets. That oil can drip down onto the exhaust manifolds and burn. It can also push oil throuugh the PCV valve, allowing ti to be ingested and burn. By revving the engine for 20 minutes you exacerbated the problem. Oil got pushed past the valvecover gasket and burned on the exhaust manifold.
I’ll bet lunch that that is what happened to you. I’ll also bet luch that you changed the PCV valve to deal with excess oil usage. Did I guess right? Or do I owe you a lunch?