slow oil pressure

So my 99 recently developed an oil pressure issue.
Only on cold starts it takes about 5 seconds to build oil pressure.
I rebuilt the motor about 10k ago and replaced the oil pressure sending unit.
It has never had this issue until the last few days , its long enough that the check gauges light glares and I can hear about a second of chatter .
I use Valvoline 10w30 and lucas and wix 51085 ( since I bought it 80k ago)
I'm thinking I may have a dying oil pump (melling high vol.)
I've used this same pump with several 4.0 rebuilds and no issues .
I also thought the mounting gasket may be pulling air .
Its only been around 30*f .
All my searches lead to low oil pressure and that's not my issue since once it builds pressure its totally normal .
Just curious if anyone has any suggestions before I drop the pan for a $100 oil pump.
Thanks .

I would put a mechanical gauge on it before I did anything else, the gauge in the dash is a little fuzzy.

http://www.sears.com/atd-tools-5550...-SPM2408507514?prdNo=2&blockNo=2&blockType=G2

i had a similar issue at one point with my '89. it would take about 4 seconds to build oil pressure after a cold start.....the clatter was pretty disturbing. temps were around 60*F. i changed out the stp oil filter for a k&n oil filter; it never did it again. i only used the stp filter because i had it sitting around-typically i use the blue box carquest filters(wix).
interesting thing was it didn't do that immediately after the oil change, it started a couple weeks later. strange...
also, i think that 10/30 is probably a bit too thick at that temp. i'd change to 5/30 and swap that oil filter for some other premium filter and see what happens. maybe you got one with a bum anti-drainback valve?

I don't think my oil is the issue , I service both mine and my wife's xj the same .
I have considered the mechanical gauge , allthough my experience with bad sending units have never acted like this . It's almost due for a service , the oil still looks nice and clean ( like usual).
Like stated in first post this has never happened before , even in much colder weather.
I'll figure it out , I was just feeling lazy this morning and thought I'd ask here .
Thanks guys .

You mentioned it clatters upon startup, so it's definitely
not getting oil. One possibility is a clogged oil pump
pick-up screen, but that's unlikely since it's newly rebuilt.

I'd change the oil filter before going further. The filter
media can collapse and obstruct the oil flow. This is
especially true since you have a high volume pump
installed.

Change the oil filter, what brand is on there now? I'm going to guess it's orange. Oil filters have an anti drain back valve built into them so that they don't drain back in the pan. It sounds like your filter is letting oil drain back, and so on start up, the pump has to refill the filter before it pressurizes the engine. It can take a few seconds for that to happen. Change the filter to a good brand like a Wix, or Napa Gold.

From what he said He has a good filter. I hunted down a oil pressure problem in another vehicle recently ended up having to pull the pan and clean the screen.

It's not a sender cause the sound of the engine confirms the oil pressure reading. Either a bad filter, a filter that self destructed under the high vol or a bad oil pump or maybe one of the gears related to it. I'd try the filter first cause it's the simplest explanation.

Orange filter ,NEVER!
It's about 500 mi. Away from a service , so it won't totally piss me off to change the oil early.
I've probably used 200 wix filters over the years and never had this issue.
But not much suprises me with parts quality anymore.
But I'm about positive now that the filter is to blame , if not it almost has to be the pump itself.

My 4.0 likes pronto filters. I've tried wix, Carquest premiums, and Bosch and I'd always get start up chatter with them.

So I changed the oil today and I found the issue.
Just inside of the oil filter I could see the diaphragm all blown out .
Just a reminder to myself to not overlook the simple stuff .
I honestly never really studied the guts of an oil filter before , wix filters have a diagram right on the box .