It does not really matter how the compression test is done as long as everybody
does it the same.
A warm engine is called for to get as close as possible to actual operating
conditions. The ring gap does go down as the engine temp goes up, so compression
will go up too.

The same with the wide open throttle. As long as everyone does it the same we
get a meaningfull test.
With the throttle open the engine will "pump up" pressure slightly faster. With
experance you can tell from how fast (how many crank revs) a little about the
engines condition. If it goes right up to 180 on the first turn then its great.
If it takes 6-10 turns then even thou it gets to 180 there is signs of a
problem. The engine should also crank slightly faster with the throttle open.

When I check my 5 cylinder Eurovan engine I start with a warm engine but with
the time it takes to unscrew the tester  from one cylinder to the next the
engine has cooled slightly and I get different readings depending on if I start
on #1 to #5 or #5 to #1. Not a significant difference, maybe 5 psi, but
detectable.

Oil is supposed to help the rings seal better and even with a flat engine can
help get a better reading and tell if its the rings or the valves. oil will not
help valve sealing at all.

Pawling NY

Cars I have: 93 Eurovan MV           Cars I had: 89 GTi 16v
                  75 Westy                                    87 Syncro
                  73 Super                                     83 Rabbit GTi
                                                                    82 Vanagon
Diesel (w 83 GTi engine)