Motor oil to use

Up front information:
I own 71 Mustang Mach 1 with a 351 W with a Summit 4 bbl carb.
I know this is not a new topic.
The car came out of the factory with a 302 2bbl. I don’t know the age of the current 351W.
A couple of us have tried to get the starter off to look for the engine number. It seems like it should be an easy removal but we can’t get it off. Don’t want to damage it.
The number on my distributor shows for a 1978.
Now my main question:
What oil should I use? I only drive a average 2 times a month and not far. I live in North Texas.
I have read I need oil with zinc (should not use additives) for older engines with flat tappets.
When looking I can find Valvoline 20w50 High Zinc.
I would appreciate suggestions and information. Thank you

Thank you.
I have been wondering when I see High Zinc how much Zinc ppm it has. Read to much is as bad as not enough.

Don’t want to run my questions into the ground.
I drive 100% of the time over 50 degrees and somtimes over a 100 degrees. I understand the zinc level in the VR-1 20w 50 has a zinc level of 1200 to 1400ppm.
The questions I have now are. Is the oil fluid enough at startup? Does it have detergent in it?
As mentioned before I have no idea how old or how many miles on the engine.
I read that you don’t want to clean built up in old engines it may cause leaks.

Reason I have been posting questions about oil.

The last time I changed the oil was less than a year ago and less than 500 miles. I used 10W 40 and hate to say it but added STP additive. The oil on the stick is really dirty/dark. Attached pictures.
Maybe slightly lighter than the pictures but not much. So why I am asking questions.