I've got a Atomic 4 manifold with a stripped-out drain plug. I've found it difficult to tap the cast iron, so I went looking for some information on how best to create new threads in a hard iron casting. Along the way, I found some interesting web pages:

Understanding tap designations:

When buying your taps, it's best to use an industrial supply house, you'll get a better quality tool. Nearly all have websites- Mcmaster-Carr, J&L, Enco, etc

Additional note on aluminum: it's best to make use of helical coil inserts (Helicoil) right from the start-the resultant connection is stronger, and is more easily disassembled in the future. Apply zinc chromate paint to the helical coil insert, and install while wet. Helicoil recommends this procedure for use around salt water- also be aware that the inserts are available in free running and locking versions.

For cutting oil, use what's at hand; I've had good results with 2-stroke premix oil; TapMagic is excellent and is available in small containers. While I haven't tried it yet, I suspect canola oil would work well, too.