Yes, if there was no oil pressure you would have internally damaged the engine. My concern is the low pressure at idle. Once off idle, (while driving) you may have enough pressure to protect the engine. Does the sound seem to come from the top end (lifter) or is it low in the engine (Rod or Main bearing)? Also, what I would like you to do is this. Go to a nationally recognized parts store see if they will lend or rent you a mechanical oil gauge so you can be sure of the pressure you have. To use it, remove the oil sending unit you just replaced and screw the mechanical pump in to the same port.
Do that and let me know what pressure you have. Also, make sure that the oil filter being used has an anti flowback valve. If it doesn't, you are basically starting the engine without oil pressure until the filter refills and begins flowing through the engine.
Let me know what you find or if you have other questions. Also, use the recommend oil weight. Today's engines have very close tollerances and require the oil to be thin at start up for quick luberication.
Joe
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Sunday, December 27th, 2009 AT 6:11 PM