Sounds like the push rods have restricted passages. Pull a push rod and see what the holes look like in them. Maybe they were gaulled up and that is restriciting the oil flow. Blow air through the push rods, too. Might be clogged with debris. What cam and lifters did you use? If you used edge orifice lifters they already meter the oil down. When the oil gets hot, it thins down and flows easier, too.