Got a situation on a fresh 292. The new oil pump (gear type) was installed when the motor was put in. 15w40 rotella oil (the old formula, with zinc). Break in has gone fine, and so did the first 500 miles or so. It seems to take 3-4 seconds to get oil pressure to my mechanical gauge I have under the dash (Sunpro, also new when I put the motor in). After that, oil pressure is fine, at idle, stop and go, WOT, cruise, hot motor--whatever. Just seems to take a few seconds to get a reading.
For the first 500 miles, I had oil pressure instantly when I turned the key. I did change the oil filter and oil (same weight/brand) after break in but still a problem. I do not think this is an oil filter deal. What about that pump? it is a Melling.
Is the oil draining back through the oil pump???
Daniel JessupLancaster, California
aka "The Hot Rod Reverend" 
check out the 1955 Ford Fairlane build at www.hotrodreverend.com