This is only a question about viscosity, not brand. As some of you may remember, I put an alarm for low oil pressure in my 84 500SEL. It was set for the following levels to activate the alarm.

Less than 45 psi above 2000 RPM.
Less than 30 psi above 1500 rpm
Less than 15 psi above 1000 rpm
Less than 10 psi above 300 rpm

It has been like this for several years now with never an alarm.
I have used 20W-50 SYN since the alarm was installed.
About 250 miles ago I changed the oil using 15W-50 Syn.
I have been getting oil pressure errors of 8 psi at 500 RPM since I changed the oil to the 15W-50, only when the engine temp is 190 to 200 deg F. I don't remember exactly what it was before, but I know it was above 10 psi at 500 RPM.
I thought that the 15W meant that the oil will never be thicker than a 15W oil when it is cold and would never be thinner than a 50W oil when it is hot. So since we are talking about viscosity when the oil is hot, why would the pressure be lower then when I was using the 20W-50?
Or, do I have another problem?

Any Ideas?
Thanks
Paul

Edit: I'm sorry I put this in the wrong place but It could apply to a diesel as well.
Edit #2: The alarm has only been in for a little over a year, not several. These things happen when you have been around as long as I have.

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84 500 SEL (307,xxx miles)