I'm sure someone here may tell me I'm wrong but what I have always been told is approx. 10 psi hot for every thousand rpm and your 20 psi at cruise puts you where you should be. With that mileage though, you are bound to have some bearing wear so that is where your pressure is bleeding off some at idle. How much below 10 psi does it go? Does it turn on the "oil" light in the dash (if it is still working that is)? Did changing the front cover and oil pump up the pressure at all? There are two main areas of wear on these oil pumps and that is the area above the gears and the cover below the gears which can also warp as well as wear. Wear or excessive clearance will show up as a loss of oil pressure but with these being new items, this should not be the case here. Blueprinting the pumps by checking the clearances is something that should definitely be done on a full rebuild but with the fact the car is already running with this pump and has "correct pressure" at operating rpm's I don't think it's necessary at this point.
I've seen small block Chevy's only hold 5-7 psi at idle that have ran for ages. I know these aren't sbc and these things are notorious for low oil pressure but on a n/a, non-performance application, as long as you've got enough oil pressure that it doesn't set off the warning light and you are not having any other noises such as lifter tick or bearing knock when idling, I would say that you've got no reason to be in emergency repair mode about this. Cars don't really spend all that much time at idle, so as long as it is getting some oil to everywhere, it's not as big of an issue as people believe. When your oil pressure starts dropping off the current levels as the rpms increase or you start to get noise at idle will be the time to worry about pulling it apart. If you want to go to 10w40 now and change to 15w or 20w for the summer it shouldn't hurt anything either but may give you a little peace of mind.