Squirt some brake cleaner spray into the hole to clear out the oil and leave the threads dry. Then clean the coil mounting bolt the same way. Apply an even coat of silicone sealer to the bolt threads and install it. Let it cure over night before starting the engine. No more leak.
I have one valve cover screw hole on my 250 that is busted through into the water jacket and with silicone it hasn't leaked in 3 years.
Teflon tape might work but on a straight bolt thread I've always had much better luck with the silicone. Teflon tape will eventually break down and dissolve in contact with petroleum products because that is what it is made from. The more solvent the petroleum, like diesel fuel and gasoline, the sooner it breaks down.
