[ylwagon]

A compression test needs to be done on a warm engine. If you run one on a cold engine that would not start, you should add a tbsp of oil to each cylinder..

True, but a waste of time (and oil) on an engine with essentially zero compression. Assuming that the diagnosis on compression was correct, that engine will sound like a whirring sewing machine when attempting to start, with none of the usual changes in frequency caused by compression bumps.

 

[ylwagon]

Yes, but an engine that does not start for another reason may have washed the oil off of the cylinder walls and shown an artificially low compression reading.

What I mean is that an engine doesn't go from say 150 PSI on all cylinders to 10 PSI or even zero because the rings are dry, as Hirnbeiss suggests. Something catastrophic occured, such as the crank sprocket key shearing.