'06 S40 T5 oil pressure issue

[RobsS40]

Bought S40 T5 a few months ago. One day suddenly heard rod bearing knock. Told story in another thread, but basically had to clean lots of galled metal off of crank pin #5, replace bearing. Other 4 bearings looked new. I don't know car's history. After fixing it, it's been great, except a very strange scenario.

A couple of months ago coming down fairly long flat exit ramp from highway, nearing an intersection and slowing almost to stop, oil pressure light came on. Engine didn't clatter, but light stayed on. Drove a few miles carefully, parked. Came out later, started, oil light went out, ran fine.

I pulled oil filter that I had very recently replaced and found a fair amount of black carbon- almost like sand, but granular- if you rub it between your fingers it turns to oil-soaked black powder. Probably crud from previous owner(s?) not changing oil frequently. Oil was still quite light amber color, so I scooped out the black carbon stuff, maybe a cc total, and put in a new filter.

Kicking myself for not installing oil pressure gauge early on, but I did finally get one installed. Pressure seems good, maybe too high, but never goes low.

I do that same highway and ramp several times a week, no problems since, until... last evening same scenario: same highway, same offramp, pressure went near zero at end of ramp, oil light on. No clatter or other noise, but being very gentle with accelerator and RPMs. Again, after it sat a while, started and drove home no problems- pressure seemed fine.

I can't figure out what could fail, but self-heal. Oil is fairly new, 5W40 full synthetic. When I changed the rod bearing I fully thoroughly cleaned the pan. All "O"-rings / gaskets were good. Removed, cleaned, and blew out oil cooler and all passages. Made sure oil gallery was clean. Also removed and thoroughly cleaned oil filter housing.

Anyone have any thoughts?

[RobsS40]

Following up: changed the oil filter, and now that I have a real pressure gauge (not just idiot light) I can see pressure is better. So it may have been a clogged filter, even though it had maybe 600 miles on it. There were some carbon sandy granules outer bottom of filter cavity. Maybe need an oil pressure gauge pre-filter so I can see the drop across the filter.