Read a recent thread on this forum, by a member who seems to make a bunch of sense, that ZDDP additive is not required for modern oils. Because ZDDP contaminates the catalytic converters, the additive package in the motor oil has been adjusted away from ZDDP. The new oils get their extreme pressure lubrication qualities via different compounds. The film strength and wear resistance of the new oils will not cause any problems with old engines.
Not only that, but this engineer's testing found that adding a ZDDP additive to modern "SN" oils makes them perform worse, not better.

 

The process I used was this:

• Removed the gauge from the car
• Charged an air tank to 35 PSI. Checked it with 2 tire pressure gauges to make sure it was 35 PSI
• Hooked the tank to the gauge using a rubber hose and clamp.
• Adjusted gauge by bending the little U-shaped brass part that connects to the needle movement
• Repeated with other pressures, like 15 PSI, 50, 70. It was fine there.
• Made sure it went to zero with pressure removed.

My experience was that the range was fine, meaning once I had it reading right at one pressure, other pressures were correct also. My problem was it was off about 30 PSI across the range. YMMV.

 

FWIW, my submerged pump setup (recently discussed elsewhere) came with anti-cavitation slots, per Bill Jenkins' "pressure balance" mod, and an enlarged bypass area, both of which can be examined here. These aren't exactly new ideas, but are apparently less than common knowledge. No flames, just food for thought...



Oh, and a filter bypass will open more often with unnecessarily high pressures, circulating unfiltered oil thru the lubricating system. So, definitely watch out for over-filtration at the cost of restriction in truly high-perf applications. There's more to proper filtering than just the micron ratings.

 

Melling makes several HV pumps. In the Select Series, they have !0% or 25% volume increase.
Oil viscosity will alter pressure slightly.
Off hand, it sounds like you have a bit of wear in the engine, but nothing to worry about.
Your running coolant temps will also affect the pressure slightly.
Don't know the history of the engine, but for as easy and inexpensive as it is, you may want to pull the pan, check a couple of caps and clean the pan and pump screen if nothing else, especially if the car is new to you.

Those are pretty normal pressures for a healthy engine using an HV pump same as what I get.
The Melling HV pumps, as well as most others, come with a 70psi relief spring, so any pressure reading of 70psi, pretty well assures that the pump is relieving, which creates an extreme amount of heat.
As long as it stays under 70psi hot, it's golden.
I've been using 0w40 for about a year now in everything and have only noticed a very slight lower psi idling cold in one vehicle.

Melling makes almost all those pumps for other branding and Melling's own anti cavitation pump is just labelled a race pump. Key is also to use as large a pickup tube as possible.

The filters that advertise as being best have a 10 or less micron rating and the race filters are up around 30 microns. If you're into high rpm, best to use a race filter and change the oil religiously, to prevent any filter bypass.