There are many variables other than oil and epoxy.
The type of ignition system significantly changes the heat generated in the coil. HEI, type ignitions properly control the coil charge time, filling coil just before use in spark. Coil charge time is typically only a few milliseconds. At idle, a ballast resistor limits the coil current, but not the time, the time between sparks is hundreds of milliseconds. The dwell time with to point adjustment reduces the charge time some, and mopar electronic without, heats coil more.
In short, modern epoxy filled coils are designed for use with mode HEI or dwell controlled systems.
Epoxy filled coils have an improved magnetic path, and a more compact design. The compact design, reduces the length of copper wire, reducing resistance and associated wiring.
The canister coils were on improvement over tar filled wooden box coils used on early Fords. Oil circulates, and serves to fill voids for self healing insulation.
Epoxy coils have special manufacturing requirements. Vacuum is drawn when epoxy is filled, to remove bubbles, that make voids. Process control is important, because once potted, bubbles are hard to identify.
