DoN. Nichols writes:

For a thread-forming tap, I'll let others
suggest the proper lube.

I form 1/4-20 and 10-32 holes every day in 6061, using perfumed kerosene
(aka WD-40).

Formed threads have a very poor quality. The crests look like little
opposing bulldozers pushed snowbanks together, with a thin gap where the
tops of the flow meet, so you have a potential false thread engagement
halfway out of "phase" with the true thread. For some cursed reason this
false thread start is the one that most easily engages when you casually
insert and start a screw. They are OK if you are doing the assembly (it
takes a certain feel to get a screw started, getting the false start, and
then backing up a half turn with a little pressure), but I avoid
presenting them to customers who have to start screws into them. But
forming certainly is faster and less prone to breakage in soft gummy
material like aluminum. The lube is important because if the metal
starts galling onto the surface of the tap, which it will after a few dry
holes, the tap will seize up and break.

One of these days I want to mill apart a cross section of a formed thread
and post some photomicrographs explaining this low quality.