This question comes up frequently. There are serious issues with epoxy filled coils.

Some of them are the heat rejection issue, oil transfers heat away from the heat source, epoxy rejects the created heat back into the source. An oil filled coil will feel hotter to the touch than an epoxy coil, because the heat is being transferred to the jacket with the oil, but held at the source with the epoxy.

If we look at the failures ofr HEI modules "for no apparent reason" in the coil in cal large HEI's, we find that soon after the coil begins to go bad from heat failure layer shorting issues, the HEI module gets over worked to ailure. In fact, it is possible that as the epoxy coil is failing, it will dispatch numerous modules before an unsuspectng tech finally reaizes the coil is the source of the failures, and replaces the coil, hopefully, with an oil filled coil. 

With my small-body HEI's I have designed them to use an easily obtained over the counter oil filled round coil, no problem. 

With any ignition system, testing of components is critical, doing it the right way is essential. To test all coils, remove them from the vehicle, find a good auto parts store with an off vehicle electrical tester, run the coil on the machine, and let it get to operating temperature. Resistance tests are no longer adequate to find an issue with a coil that has failures occuring only when a coil gets to operating temperatures.