AFAIK the most important factors in using oil to transfer heat are:
1. volume
2. motion (is it passive, or does it drain away constantly)
3. area (how large a heated surface is washed)

Oil from the rockers etc. that can "find" a narrow deep channel to the drains is going to do less than oil that must pass over the exhaust guide bosses, exhaust port rooves, and spring bases first. Examine your head's upper surface - sometimes there are obvious areas where a simple piece of sheet metal creates a longer path.
Some engines have drains adjacent to hot spots - here, drilling the drain slightly oversize 1/4" deep and inserting a thin tube 3/8" tall (old conduit, etc.) makes the oil pool out and cover the hot area 1/8" deep before reaching the drain opening. This is even possible with some pushrod holes. Remember to analyze this as the engine sits in the chassis, both as to front/rear and left/right.