Oil pressure warning light ON...!?!

[GPZ1100ABS]

I'm a little bit troubled to say at least.

After completing the winter overhaul I had the engine started for the 
first time since November. It fired right up and ran smoothly at fast 
idle. After a minute or so the mechanical sound of the engine changed a 
little bit - not much,  just barely noticeable. It was not rattling or 
noisy or anything, just a little bit different. Not from the top end, 
more like a change in the crank case. Not overly concerned I decided to 
take it for a spin on the suburban road, just a couple of hundred 
meters. During this the oil pressure warning light flashed a couple of 
times, then a couple more times at a longer duration, then finally it 
came on permanently. I stopped to check for oil leaks and found none, 
plenty of oil in the engine. Fired it up again, the red light remained 
on, shut down again.

Of course I should have changed the oil and the filter before putting 
her away for the winter. On the other hand the oil quality should not 
degrade so much during winter as to cause engine failure at startup 
months later?

I have drained the oil and taken the filter out. Tomorrow I'll put in a 
new filter and add fresh oil and see if the problem remains. But given 
the little change in the sound of the engine I fear that something 
mechanical has happened.

The only work I have done on the engine this winter was valve adjustment 
- something I have done more than a dozen times, and I didn't drop 
anything in the crank case nor left any piece of cloth or anything else 
in the top end. So what could it be? Can the oil pump quit all of a 
sudden? Any ideas? The engine has run 158.000 km (almost 100.000 miles), 
but last year it was still running smooth and with low to moderate oil 
consumption (approx. 1 liter pr. 10.000 km)

Ped
'96 Black Stealth" GPZ
Denmark

PS: Fortunately I have George's engine sitting in the garage, but I'd 
like this one to run just a little bit longer...