Pool and Spa
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400 mV should be enough to hold the pilot valve open once you release the knob/button. The fact that the pilot goes out would indicate a problem with the gas valve. It will probably need to be replaced.
Yes you can use any universal millivolt valve as long as it has that BTU capacity and the tappings in and out are big enough (most come with bushings to reduce the size if needed) . They are all standing pilot.
Here's an inexpensive optionhttp://www.supplyhouse.com/Robertshaw-700-506-3-4-x-3-4-Millivolt-Combo-Gas-Valve-300-000
380 mV is borderline but should still be enough to keep it running. More likely one of the devices in the thermostat circuit is shutting the unit down. You need to jump out each device (temp sensors, flow switch,etc) one at a time and see if the unit runs when one of them is jumped out. If it does then that device is the culprit. It's also possible the device is functioning normally and you need to correct the issue that's causing it to shut the unit down