The change in pressure over time you describe may indicate that you have a leaky injector diluting the oil with gasoline. You can send the oil for analysis to Blackstone test labs for $22.50 and find out for sure.
[edit: thanks for the correction, willwren]
I run 15W-30 in my '95 @ 155K miles. The oil clearances on my bearings have increased over the years to the point where 5W-30 would be unable to build normal pressures. Winter temperatures where I live get to about -15F, but I have not had trouble starting with 15W-40.
How hot does the car run? A 180F thermostat will lower engine temperature, increase oil viscosity and increase oil pressure. A transmission cooler will also increase engine oil pressure because it lowers coolant temperature coming from the radiator. I know, it doesn't seem like it should if the cooling system and thermostat are working properly, but experience has shown me that it does.
I am a firm believer that older, worn engines benefit from heavier oil. Many years ago I kept a very tired Fiat 124 running for a couple of years on straight 50-weight oil and STP. When I purchased the car for $50, the oil pressure was so low with regular 10W-30 oil, it did not register on the gauge. You pay a price in fuel economy with heavier oil, but gain in engine (bearing) protection.