Your oil pressure is lower because your oil is thinner and flows more easily through the engine. Thinner oil may not provide intended lubrication which can contribute to early wear. Many actually recommend going to 5W30 if you live in icy conditions, just to help the oil get through the engine until it is warm.
5W20 may be pushing it, as at running temps your oil is going to be thinner. For those who weren't aware, the numbering/weight for oil is the thickness/viscosity at a certain temperature.
SAE30 is constant at all reasonable temperatures (cold or hot)
10W30 has a weight of 10 at cooler temperatures (such as freezing) but has a weight of 30 at 210 degrees F. The W stands for "Winter".
This is done to get oil through the engine after sitting off, and as the engine warms up the oil thickens to provide more lubrication/protection.