Oil light comes on at low rpm

The problem filter was a KN-401 from K&N Performance, which is what they gave me at Advance Auto when I told them the make/model/year of my bike. I replaced it yesterday with a Fram CH6012, which is the same filter the PO used. 

Update: Might not be out of the woods just yet. The oil light came back on.

I was riding the other day and everything seemed fine. The bike had warmed to operating temp and there were no warning lights at idle. Then I got to my destination, killed the motor, had a chat with my friend, got back on the bike, and when I started the engine, the light had come on. Again, it only stayed on below 1500rpm. I was going to test the pressure when I got home, but by the time I was back in the garage, the light was no longer coming on. 

I rode again the next day, and had a similar situation. First, the light would stay off at idle for the first hour or so of riding, then it would come on. Then it would turn off after a while, then come back on later. It's been a lot less consistent since I changed the oil filter. The light was still on when I got home, so I checked the pressure and it was a bit low - about 22psi @4000rpm. 

Any thoughts? 
 

It´s time to focus your attention on the oil pump and filter screen.

I've drained the oil, removed the filter, exhaust, and clutch pack, and I'm getting ready to pull the oil pan and pump. The FSM says to replace the pump cover gasket (part # 16145-004) during reassembly, but I'm having trouble finding the part online. I checked Partzilla and Z1Ent, but neither of them seem to have it. My google search didn't turn up much either. Same for pump rotors (16154-003). Does anyone know where else I might be able to find these parts? 

I've also heard that the oil pan can be a PITA to remove, so I'm open to any tips for that process as well. Thanks!

I’m chiming in pretty late into your ordeal. But these are the only insights I can provide. 
The oil sump is insanely simple to remove. Remove bolts, oil cooler tubes (if you have an oil cooler that is) and the oil filter cover and the oil sump will right off. The old sump gasket may try to hold it on, but give it taps with a rubber mallet but don’t dig at it with a screwdriver. If it is held on by TTV or the like, then someone else has been in there and that is likely the pickup screen is filled with rtv strands. 
Use a impact driver of some sort with the Phillips tip to remove the oil pump screws. You will destroy them trying to use anything else. 
I always search ebay for old gaskets and almost always have found someone selling one. They used that exact oil pump in the 550/600/650/750 from the 70’s to the 90’s and I have never heard of someone finding one that was bad. But if it sucked something up (rtv) or the screen is covered (rtv) and starved for oil then anything is possible. I would honestly just buy a used pump as the chance of getting another bad one is virtually impossible. 
But first……check the oil relieve valve that is attached to the oil sump. If it has smutz stuck in the plunger you may be leaking oil pressure back into the oil sump and not getting pressure to the main oil gallery. That could be your entire issue right there. 

I managed to remove the oil pan by leaving 4 bolts in place just a few threads deep, then holding a 2x4 against the pan and giving it a few good whacks with a hammer. The gasket came off pretty clean, minus a couple small problem spots around the bolt holes. I could probably scrape those off with a razor, right?

I took a look at the oil pump screen, and it was definitely dirty, but I don’t know if I’d go so far as to say it was clogged… there are some small particles, and that large object is a piece of a leaf. The streak across the middle is just due to the lighting. Does this look like enough debris to be causing the drop in pressure?

 

Removal of the oil pump mounting screws was a colossal pain in the butt, as they were staked in place. Even with my Vessel JIS Impact screwdriver, they wouldn’t budge until I upgraded from my 20oz hammer to a 4 lb mini-sledge. When I eventually reinstall the pump, will I need to stake the screws again? Or is it enough to just use blue Locktite?

 

I checked the pump rotor clearances with a feeler gauge, and they were well within the service limit of 0.3mm. The rotors and gear don’t appear to have excessive wear, but I will let you folks be the judge of that. I preemptively ordered new rotors, so should I just replace them anyway, since I have them?

Side1:
 

Side 2:
 


The pump body and cover appear to have some scratches/grooves. Does this indicate any significant damage?

 
 
  

The 90mm O-ring that sits in the pan, around the oil filter hole, seems a bit worn – notice the fraying on the right side of the image. I’ve ordered replacements for all the O-rings and gaskets that I’ve had to remove. Could a poor O-ring be contributing to low pressure?



The pressure switch lead looks a little dirty, so I’ll clean that up when I reassemble everything. Dirty electrical contacts could potentially explain the oil light coming on in error, but it wouldn’t explain the low pressure readings, unless my gauge is incorrect.

Lastly, I checked the relief valve per @Daftrusty’s suggestion, and the spring mechanism appears to be working properly.Again, thanks to everyone for all the help so far. I’m hoping we’re getting close to a solution, but if the problem isn’t being caused by anything mentioned above, what could our other culprits be?

I had a similar problem with my wife's mini. I replaced the oil pump, which looked a lot better than yours does, and the problem got a bit better. I still have the problem though and the next step is to check the crank & big end bearings. If the clearance is way out it will be hard to build up oil pressure at low RPMs. 
I would replace that pump and hope the problem goes away - the pump body seem too far gone to just put it back in. If the pump and the relief valve are all good and the problem still persists, then I'd be checking the camshaft, crank & big end bearings

Your very first post, according to the factory service manual, does not seem to be an issue if the bike is at idle.
It is an issue if the light stays on above idle.

wrote:

Your very first post, according to the factory service manual, does not seem to be an issue if the bike is at idle.
It is an issue if the light stays on above idle.

I've read that section as well, which is why I wasn't too worried at first. Initially the light was shutting off around 1300-1500rpm, but more recently it had been staying on up until 2000rpm. Also, my pressure tests indicated about 20psi at 4k rpm, which is lower than the ideal range listed on that same page of the FSM (28 - 36psi). 

I've had issues in the past with my oil pressure light coming on at idle.
That would happen once the bike got hot. I attributed the light to the oil thinning out and low rpm's.
I was running 10w-40 during spring/summer. I have since stuck with Kawasaki's 20w-50 oil and the oil light has stayed off.
Initially, I figured you were having the same issue.

In my opinion, that oil pump "looks" OK... Small striations in the rotor contact faces are "normal".
However, you will have to verify the measurements set out in the instructions. (See attached pics)
The pump cannot work effectively (create pressure) if clearances are bigger than spec.




If the measurements are ok, then it is clear that there is a pressure drop due to increased clearances elsewhere.
The first thing I would then check is if the oil pressure orifice could have been omitted, or incorrectly installed by the previous surgeon! 
I'm not sure what the effect of a hardened / faulty o-ring would be (the "oval" shaped one that surrounds the oil pressure orifice in the cylinder block) but it would seem logical that if oil was bypassing the orifice, the pressure would potentially be reduced.

 

H.

wrote:

I've had issues in the past with my oil pressure light coming on at idle.
That would happen once the bike got hot. I attributed the light to the oil thinning out and low rpm's.
I was running 10w-40 during spring/summer. I have since stuck with Kawasaki's 20w-50 oil and the oil light has stayed off.
Initially, I figured you were having the same issue.

I'm a little nervous to try a different oil weight, since both the FSM and the oil filler cap say to use 10w-40. I've read in another post that you should only increase oil weight if you're riding pretty much exclusively in hot summer weather - and I live in New England, so that is not the case