TOPIC:

While the bike is warming up, there is no problem. But after about 15 - 30 minutes of riding my '82 KZ550A, the oil light turns on whenever I come to a stop and the revs drop to near idle (idle spec for my bike is 1k - 1100rpm). Once I twist the throttle, the oil light turns off as soon as the rpm exceeds 1500. The light stays off while I'm riding, then illuminates again right when the tach drops below that magic number 1500. 

I did an oil change about 250 miles ago (10w40 conventional), and I topped it off just the other day when it looked a little low. The sight glass is more than half full, but I still have this issue. The bike seems to be running fine, and I don't notice any difference at low rev aside from the light itself. Does anyone have an idea as to what might be causing this?

Need to do a pressure test. The smaller fours have high oil pressure. Could be a clogger filter screen or faulty sensor unit.
Steve

Thanks Steve - How do I do a pressure test?

There should be a test port on the side of the engine.
Steve

If you can not do a pressure test, just replace the sensor. They are cheap and have at least a 50/50 chance of keeping the light off.
 

.....and if you had to top it off because it was 'low'. ....... Make sure you are not loosing oil for real, other than external leak, like valve stem seals.......

I was able to get hold of an oil pressure gauge. While cold, the oil pressure measured 75psi @ 5000rpm, which is within the range listed in my manual (63-85psi). After warming up for about 7 minutes, the pressure gauge still read 65-70psi when revved to 4000rpm, which is definitely higher than the range from the shop manual (28-36psi). The bike was warm enough that it was comfortably idling at 1100rpm, but not warm enough that the oil light came on. It usually only comes on after riding for a while, but I'm hesitant to ride it while it's having these issues - I noticed a bit of a burning smell when I was riding the other day, which made me nervous. Do my pressure readings tell us anything, or should I try to test the pressure while the oil light is on?

I hate that when the manual tells you what the reading should be at 4,000rpm, and does not say what it should be a idle. They do the same thing with battery charging voltages.
Go for a ride and see what pressure the light comes on. Since the minimum pressure is 28psi @ 4,000rpm, I would expect idle pressure to be less than half that.
Have you tried researching the pressure switch and see what PSI it should activate?

I haven't been able to find anything about the psi at which the pressure switch is supposed to activate, aside from those tables I posted from the manual 

Basically, what the manual says is;

When engine is cold, the test is for the pressure relief valve:  If it is with range 63-85 psi, then valve is OK.   If is above, valve is stuck close, if it is below, valve is stuck open.

Then remove gauge and run bike around the block, just to get it to operating temperature...and being air cooled, it may take a few blocks to get it there.  Then when hot, install gauge again...and start engine and at 4000 rpm and 90C or 194F oil temp;   (I know it is very scientific)

If it is within 28-36 psi, then pump is OK.   If low, then is not.

.......but agree the manual does not say anything if pressure stays high below max.    I'd say so far, oil pump and relief valve looks OK, and oil pressure switch is most likely suspect.

I'll take it for a ride around the block and check the pressure when I have time later today. If the pressure switch is the culprit, that's probably the best case scenario, but it doesn't explain the burning smell I noticed the other day. 

Mama Kaw's big-block roller crank motors (Z1, KZ1000 etc.) spec oil pressure at 2.8 lb./sq./in. @ 3,000 RPM with 140 Deg./F oil temp.  Real world pressure is somewhat higher when oil is that cool.  We've seen pressure on  Z1's approaching 30 lb./sq./in. after a cold start at road RPM (3,000 ~ 4,000), eventually dropping to 3 ~ 3.5 lb./sq./in. at road RPM after 10 - 15 continuous road miles.

What does that have to do with a KZ550?

The three quarts of motor oil in a KZ550 represents a significant thermal mass.  Even though the jugs & head might be hot after idling for a few minutes, the oil in the crankcase and the crankcase / oil pan vessel itself will still be relatively cool.  Also consider that the oil is held in the crankcase / pan vessel that has much finned surface area from which to transfer heat as the oil drains from above.  Much time is required to dump enough heat energy into 3 qts. of dense oil plus the very significant thermal mass of the motor itself to raise the combined temperature to operating range.

A comment made in the OP "... after about 15 - 30 minutes of riding my '82 KZ550A ..." represents sufficient time to heat the oil plus the crankcase / pan to full operating temp, at which time pressure testing would likely be most accurate.  Idling for 7 minutes, probably not.

One thing we don't see in the pressure spec. kzmax posted above is, at what oil temp is the pressure specified?

Good Ridin'
slmjim & Z1BEBE