I don't have the answer for you, but I can tell you that my truck does the same thing. On cold mornings my oil gauge will read between 20 and 30 at start up but drops to about 5 when warm. At cruising speed I'm lucky to get 10 psi. So I'd say you're doing pretty good.
I also get black water out the tailpipe until it's warmed up. The black water is because she's running rich. She's running rich because the choke is pulled out, which is necessary until she warms up. Once she's warm and you push the choke in, it returns to a 'clean burn' condition and the exhaust clears up. It used to bother me at first, but my truck hasn't gotten any worse over the past 9 or 10 years as my daily driver. If I start getting excessive black water at start-up I know it's time to adjust the valves and carburetor and change the plugs, which is about once a year.
You're doing the right thing to notice and pay attention to the things your truck does. But don't let it stress you, she's almost 80 years old. Only start worrying if she starts doing something different or unusual.
