Oil pressure concerns

1974 318. 40000 on factory bottom end. Running Lucas 10w30 with Wix filter. 4 extra holes drilled in oil filter mounting plate. Oil pressure is 65-70 cold and 50-55 cruising at 2300 warm as per aftermarket gauge. The two concerns I have is that pressure at 800 rpm idle with 190 coolant temp is only 20 psi. In gear drops to15-18. Pressure comes right up as soon as you touch the gas. The other thing is that on a cold start, the gauge takes 2-3 seconds to move. Acts much like first start after an oil change when filter isn't saturated yet. I ran about 1000 miles last summer using 10w30 synthetic and I'm concerned it may have super-cleaned the inside of the engine and sludged up the oil pickup. Maybe I'm just paranoid. I do have another mech gauge that I could hook up temporarily to see if it shows same as Autogage. Thanks for any opinions.