You need to pull the pan. What is likely happening is the pickup is getting clogged or pump is weak. Oil change can help but if that doesn't do much then you still need to drop the pan and now it cost you an extra oil change.
Also going to thicker 20w50 doesn't do much at all. If you look at oil's kinematic viscosity at different temperatures, when cold the stuff is molasses and starves your engine while when warm it's so close to the same viscosity as the 10w30 specs that it's an effort in futility. Oil pressure isn't altogether that important anyways, oil flow is and the 2 are not necessarily proportional. Things like Lucas Oil Stabilizer for example, all they are is a tackifier that is 10x thicker than a 10w30 when at temp. The biggest thing thicker oils do is increase cold temp wear. Cold temp wear accounts for, on average, 90% or so of your engine's wear. You're better off sticking to regular 5w30/40. 20w is stupid to put in for the cold temp reasons and going to 50 on the hot side isn't gaining you anything
Pulling the pan isn't hard either. Yes, it sucks laying on your back but it's just 1/2" bolts and the pan comes out. Another thing that can help is pulling the converter dust cover for some extra wiggle room. Also, new oil pump will need primed BEFORE the engine is started. Common method is to use a drill through the distributor hole. This will also require setting the fuel sync.