Subaru EJ22 oil pressure light on cold start?

I have had this car a a long time, its a 96 Outback, and the engine in it currently is from a 98 Legacy (~130k on the engine, it came from the junkyard out of a rolled car, the cylinders were full of oil when I got it).  I have put about 40k miles on the car with this engine in it, and every time I start it the oil pressure light will flash en extra time then it's fine after that.  Recently, something broke in the front suspension so I have left it sitting for a few weeks until I can determine if it's repairable, and I started it up once and it definitely had lost oil pressure while sitting and it clattered loudly for 3-4 seconds before going back to normal.  Any idea what could cause this?  Assuming I can repair the suspension I would like to fix the engine rather than just running it until it dies, I do have another extra engine if necessary but that would be quite a bit more work to get ready to use. 

Missing drain back valve In the oil filter?

Streetwiseguy said:

Missing drain back valve In the oil filter?

Factory oil filter, and it has had this issue for multiple oil changes. 

You really need to stick a gauge on it to see what it's doing.

I had a Jetta 16v once that lost pressure. The pickup screen was mostly covered with a waxy residue I had to scrape off. This, despite regular oil changes during my tenure but it had well over 100,000 on it.

Do EJ22s even use oil pressure?

In reply to Duke :

Just enough to push it out all of the seals lol.

To add something maybe helpful, when I had a 96 Legacy with the EJ22 it surprisingly leaked oil! If I let it go too long it would get to the point that the hydraulic lifters would collapse and all the extra noise would trigger the knock sensor and it would pull all the timing. I'd top it up and 50 km later everything was fine again, I'm embarrassed to admit that happened probably 3 or 4 times to me and that engine was still running just fine when the body disintegrated around it. So, I'm in the keep oil in it and drive it camp and not worry too much about it but only time will tell of that is good advice or not.

The oil pans tend to rust and leak   if you ever need to pull the plan the clean the pick up tube.  Otherwise keep it at the full line or a touch higher.  Use a good syn oil 0-40 or 5-30, 0-30

Sounds like the lifters simply bled down and refilled in the 3-4 seconds.   Unless I'm missing something (such as it doesn't have hydraulic lifters)

I think the bolts on the oil pump backing plate have a rep for loosening. Can cause a variety of symptoms. By '98 think the EJ22 had lost the HLAs and gone to manually adjusted valve clearance, so I don't think bled down lifters were causing the noise, assuming you have the correct year for the junkyard motor. 

Yeah it's a solid lifter engine, the oil pan was replaced not long ago (I replaced it with an aftermarket one when I put it in but the drain plug wouldn't seal after a few oil changes so I had to pull it back out and put a new factory one on).  It's entirely possible there could be an issue with the oil pump, I did pull it and change the crank seal before using the engine but I didn't re torque the bolts.  Checking means taking half the car apart but I may have to do that.

I've had a number of EJ22 powered cars and have never done the re-torque the oil pump backing plate screws thing myself but I have read about it being an issue. I'm in no way saying it's the issue with your car- just that it's a possibility. If you google "EJ22 oil pump screws" you'll pull up a few threads. Some folks claim that every one they check is tight, some folks say that they have found multiple loose screws. Good luck!