I have same 103 engine in my '91. The pressure behaves exactly as you described, and has for the last 13 years.
First time I remember it going to less than "1" was the first hot day, when the car was one year old with less than 10k miles.
I use 10w-40. Thicker oil might raise it a bit when hot.
"1" corresponds to 15psi, so it's probably dropping to about 10-12 psi at slow idle in gear. The small-block Chevy experts say 4psi is sufficient at idle, with approximately 10 psi for every 1k rpm being a good rule of thumb to protect a Nascar engine.

So, fuggitabout it.