416 low oil pressure at idle

The oil reference will only show you if the pressure is higher and lower due to viscosity, when the engine is cold.

If you swapped the oil, pressure will be closer between cold and hot, but you may need 20w/50 oil if that is how your builder set spring pressure and clearances up.

If your lifters are showing groove out of the bore on high lift, that's where all of your pressure is going. When that oil warms up and drops viscosity, it will creep even more out of those grooves, because it flows faster.

Remember, pressure is the opposite of flow. You need pressure to create motion/flow, but they are equal opposites in forms of measurement.

If what MRL is saying is happening with your engine, it's likely ok at high viscosity/ cold, because the oil is not thin enough to find it's way out of the lifter bore fast enough to drop pressure significantly.

Your crank is likely getting hosed by the lifter bores from above, too, which is not helping balance or performance.

If the sides of the lifters are exiting the bore to the point of any ridge showing, it could potentially broach the bore or create odd tension in the lifter bore, due to the lifter bore no longer supporting the area of the lifter, below that groove/ void area, and when it goes back up into the bore, the edge can potentially contact the bore untrue and broach it or even break a lifter, under high load.