Check engine light w/ normal oil pressure
 

1) I recently purchased a well maintained 1988 944/2 with 168K miles, and I love it. Last week the check engine oil pressure light began coming on as the engine warms to operating temperature and oil pressure drops below 4 bar when idling down. Oil pressure range that I'm getting, according to the gauge, is 4.5-5 bar when cold, 2.5-3+ bar when fully warmed at idle. I believe that these pressures are where they should be, IF the gauge is indicating correctly. Any suggestions as to why the check engine/oil light is coming on?
2) The car is due for an oil change. The previous owner used Valvoline 10W-40W Max Life synthetic blend. Is this a good oil for a 944, or should I use a fully synthetic (e.g. Mobil 1)? I plan to use Mahle oil filters. Any suggestions?

Because the car is a quart low on oil (or more)

Doc; Thanks for your reply.
My son thought the same thing (quart low - or more), so he added a quart of oil. Now the dip stick reads above the max line, and the oil pressure warning light still comes on as described in my original message. Any other thoughts?

Is it the oil pressure warning, or the oil level light? The only oil pressure warning is when the needle drops below 1bar and the little red bar at the bottom lights up.

The check oil light is a level sensor issue. If the wire going to the sensor is shot, it will stay on. I have seen mice chew through that particular wire on more than one occasion. Or maybe it is just unplugged.