You might get away with 40 PSI on a machine torch and large hoses but I find that 50 to 55 works a lot better when cutting by hand with the usual equipment most of the guys here are going to have, however if you are super steady you may be able to do it, most people will not be able to. Besides that's way bigger than anything likely to be found for what anyone here will be using. 40 PSI is way too much pressure for 1/2" or less and for automotive work like what we normally talk about here 25 lbs is plenty with a, for example, Victor no. 1 tip or slightly higher with a smaller tip, it in fact will give a better cut than the higher pressures on these thin materials in addition to saving Oxygen. Again there simply is no one Oxygen setting for all around use but if that's the way you do it and it works for you then by all means keep doing it that way. For you guys learning to do this try using a tip chart for the particular brand of tip/torch you will be using or simply adjust the Oxygen down to the lowest PSI that gives a clean cut for you. Some of the smaller rigs with the 1/4" hoses may need a slightly higher pressure setting than the bigger torches with larger hose but the difference will be slight.
Here is a handy chart I have used as an example before and it works quite well, 40 PSI certainly is used as are many other pressure settings.
These are just good starting points however and good cutting skills will determine what variations are required when using different tip sizes than the chart shows. Very few people would have an exact optimum tip size for for every thickness of metal likely to be encountered so as an example then the chart shows 1/2" plate at 30/35 PSI with a no. 0 tip but using a no.1 is just fine at about 25 to 30 PSI, a no. 2 will also give a good cut on 1/2" with about 25 PSI on the same plate thickness but that would be about the largest practical size to use on plate of that thickness. Certainly larger or even smaller tips could be made to work but anything out of that range starts to push practical limits, same principle applies to thicker/thinner metal. This is why there is no one setting for everything and telling someone who is just learning that there is will be getting them off to a bad start. Generally adjusting the Oxygen pressure before starting a cut is just part of the process and after you do it a while it becomes quite simple to look at the size of the tip and the thickness being cut and set the pressure according to the task at hand. A "one pressure for everything setting" may work ok for some things, make a poor cut for some and simply waste Oxygen on others so why not just learn to do it right? Nothing to it really and it simply becomes second nature to know how much pressure will work best for what after a person does this for just a little while.