On Sun, 25 Jul 2010 05:33:00 -0700, Bill wrote:
Have the proper outlet, wiring, and circuit breaker installed!

Let's look at this from a "safety" standpoint, which is your point.

The "proper" way to do this, all will agree, is to spend more than the
compressor is worth to install a 20-amp dedicated NEMA 6-20R receptacle.

However, from a safety standpoint, if anyone can show me how plugging the
20-amp NEMA 6-20P through an adapter to the 30-amp NEMA10-30P receptacle is
inherently more dangerous, then I'll listen.

Of course, I do realize that the "equipment" won't be protected to 20 amps
(it will be protected to 30 amps); but I'm not at all worried about the
30-year old compressor burning up.

The "house" wiring is still protected to 30 amps, which is what matters.

And "safety" is the same (as far as I can tell) since we're using exactly
the same two hots and the same ground/neutral.

Or did I miss something critical in that analysis?
Advice always welcome.